Preheat the oven to 350°F.
Add the ground beef to a skillet over medium heat. Cook until nicely browned. Approximately 3-4 minutes.
Add onions to the skillet along with salt and pepper and red pepper flakes, if using. Allow onions to soften. Approximately 1-2 minutes.
Add potatoes to the skillet and stir well. Allow to cook so the potatoes begin to soften. Approximately 3-4 minutes.
Add peppers and drained tomatoes to skillet and stir well.
Stir cornstarch into liquid and add liquid to skillet, stir well, and then turn off the heat. If your skillet is not oven-proof, transfer the contents of the skillet to an oven-proof baking dish. (See video.)
Top the casserole with cheese and place the skillet (or oven-proof baking dish) on the middle rack of the oven.
Bake for approximately 15-20 minutes until the cheese is bubbly and a golden brown.
Remove the skillet (or oven-proof baking dish) to a heat-proof surface and allow to cool for about 5 minutes before serving.
Well wrapped, the hamburger casserole will stay fresh in the fridge for 3-4 days. To reheat, cover with aluminum foil and place in a 350°F oven for approximately 15 minutes or until heated through.
Well wrapped, a hamburger casserole can be frozen for up to 4 months. To reheat, defrost in the refrigerator, cover with aluminum foil, and place in a 350°F oven for approximately 15 minutes or until heated through.