• Step 1 — Cook the sweet potatoes
• Place the peeled, cubed sweet potatoes in a large pot and cover with cold water by about an inch.
• Bring to a boil over high heat, then reduce to a simmer and cook until a fork slides through easily — about 15–20 minutes depending on the size of your cubes.
• Drain thoroughly in a colander, then let them sit for a minute to steam off excess moisture.
• Pro tip: Start in cold water so the potatoes cook evenly; overcooking will make them water-logged, and undercooking makes mashing miserable.

Step 2 — Mash and flavor the filling

• Transfer the drained potatoes to a large mixing bowl.
• Add melted butter, brown sugar, beaten eggs, milk, vanilla, cinnamon, nutmeg, and a pinch of salt.
• Mash with a potato masher for a slightly rustic texture, or use a hand mixer on low for a silky, smooth base. Scrape the bowl with a rubber spatula and taste—adjust sweetness or salt as needed.
• If you like a lighter, fluffier casserole (my go-to when I want the best sweet potato casserole), finish with one or two quick pulses of the mixer — but don’t overwork it.
• Pro tip: Don’t overmix with a stand mixer or high speed — sweet potatoes can get gluey if beaten too long. Stop when the filling is smooth and creamy.

Step 3 — Make the topping

• In a medium bowl, combine the flour and brown sugar.
• Cut in the softened butter with a fork or your fingers until the mixture is crumbly.
• Stir in the chopped pecans so you have a nutty, streusel-like topping. If you want marshmallows, keep them aside to add later.
• Pro tip: Rough-chopping the pecans gives better texture — you want crunchy little bites, not dust.

Step 4 — Assemble (bake now or prepare for the freezer)

• Grease your 9x13-inch baking dish lightly with butter or nonstick spray.
• Spoon the sweet potato filling into the dish and smooth the top with an offset spatula or the back of a spoon.
• Sprinkle the pecan streusel evenly over the top. If you plan to add marshmallows, either add them in the last few minutes of baking or do a half-and-half topping (streusel on one side, marshmallows on the other) so everyone’s happy.
• If baking now (simple sweet potato casserole):
• Preheat oven to 350°F (175°C). Bake uncovered for 25–30 minutes, until the filling is hot in the center and the topping is golden. If using marshmallows, add them in the last 3–5 minutes and broil briefly to toast — watch carefully so they don’t burn.
• If making a freezer sweet potato casserole or freezing sweet potato casserole for later:
• You can freeze this casserole either unbaked (my preference) or baked. For an unbaked freezer sweet potato casserole: cover tightly with plastic wrap, then wrap with foil and label with the date. It keeps well for up to 3 months. For a baked version: cool completely, then wrap tightly and freeze.
• Pro tip: For freezer storage, double-wrap (plastic then foil) and label with bake instructions and date — it saves so much mental energy later.

Step 5 — Baking from frozen / Thawing & cooking times

• From frozen (unbaked): Remove plastic wrap/foil top layer if possible, place the still-covered casserole in a cold oven, set to 350°F (175°C), and bake for 60–75 minutes, then uncover and bake another 10–15 minutes until the center is hot and the topping is golden. If adding marshmallows, add them during the final 5–10 minutes.
• From frozen (baked): Thaw overnight in the fridge if you can, then reheat at 325–350°F for 30–40 minutes until warmed through. If reheating from fully frozen, plan on 45–60 minutes covered, then 10–15 minutes uncovered.
• From refrigerated (made-ahead, not frozen): Bake at 350°F for 25–35 minutes until hot in the center.
• Pro tip: Use a thermometer if you’re unsure — the center should register around 165°F (74°C) for safe reheating and perfect warmth.

Step 6 — Finish, reheat, and serve

• If you’re toasting marshmallows or crisping the topping at the end, switch the oven to broil and watch closely—30–90 seconds can be all the difference.
• Let the casserole rest 5–10 minutes after baking so it sets up and slices cleanly.
• For leftovers, reheat portions in the oven at 325°F until warm, or microwave single servings (covered) for about 60–90 seconds, stirring halfway.
• If you want to turn leftovers into a sweet potato breakfast casserole later, try stirring in a beaten egg or two and some cooked breakfast sausage or chopped apples, then bake until set — delicious the morning after.