Base method: In a medium microwave-safe bowl, stir together 1/2 cup rolled oats, 1/2 cup milk (or specified milk substitute for that variation), 1/2 cup water, and a pinch of salt; add any mashed banana or other mix-ins listed for the chosen variation.
Microwave the bowl uncovered for 2 1/2 to 3 minutes, or until oats are cooked and have absorbed most of the liquid.
Remove from microwave and stir in the finishing mix-ins listed for the variation (for example almond butter, honey, and almond extract for the almond-banana version).
Let the oatmeal rest a few minutes to thicken and cool slightly; thin with a splash of milk if desired.
Peanut butter powder variation: Use 1/2 cup milk, 1/2 cup water, and oats; after microwaving stir in 2 Tbsp peanut butter powder and 1 tsp honey, then dollop and swirl 2 tsp Nutella and top with 1 Tbsp chopped peanuts.
Coconut-banana variation: Use 1/2 cup canned light coconut milk in place of milk and include mashed banana; after cooking stir in 2 Tbsp shredded coconut, 1 tsp brown sugar and 1/8 tsp coconut extract, then top with 2 Tbsp chopped mixed nuts.
Chocolate-banana variation: Add 2 tsp cocoa powder with the oats before cooking; after microwaving stir in 1/4 tsp vanilla and 2 tsp brown sugar, then top with 2–3 tsp mini chocolate chips and sliced strawberries if desired.
Maple-pecan variation: Prepare the base oats, then stir in 2 tsp brown sugar or maple syrup and 4 drops maple extract to taste; top with 2 Tbsp chopped pecans.
Apple variation: In a microwave-safe bowl, heat 2/3 cup chopped apple with a little water for 2 minutes; stir in oats, 1/2 cup milk, a pinch of salt and 1/4 tsp cinnamon and microwave 2 minutes more. Stir in 1 tsp brown sugar (or maple syrup), 1/8 tsp vanilla and 1/2 Tbsp butter if using; top with 1 Tbsp dried cranberries and/or chopped walnuts.
Pumpkin spice variation: Stir 1/4 cup pumpkin puree and spices (1/2 tsp cinnamon scant, pinch ginger, pinch nutmeg) into the base oats before cooking; after cooking stir in 1 Tbsp maple syrup and 1/4 tsp vanilla and top with 2 Tbsp chopped pecans.
Berry variation: Prepare the base oats, then stir in 1/2 cup mixed berries (thawed and drained if frozen), 2 tsp honey and 1/8 tsp vanilla; let cool slightly and top with 2 Tbsp half-and-half or cream.
Stovetop option: Any variation may be cooked on the stove—follow package directions for cooking rolled oats and add the same mix-ins as directed above.