Pour the jar of tomato pasta sauce into the skillet with the browned beef. Stir well to coat the meat and bring everything to a gentle simmer for 2–3 minutes so the flavors start to come together. Turn off the heat.
Scatter the sliced green bell peppers evenly over the bottom of the ungreased 9×13-inch glass baking dish. The peppers will soften and sweeten as they bake, so don’t worry if they seem like a lot at first.
Pour the hot beef and sauce mixture over the bed of bell peppers in the dish, spreading it out with the back of a spoon so it covers the peppers in an even layer.
Sprinkle the shredded cheese evenly over the top of the beef mixture, making sure to go all the way to the edges so you get those nice browned, melty cheese edges.
Cover the baking dish tightly with foil and place it on the center rack of the preheated oven. Bake for 25 minutes, until the peppers have started to soften and everything is bubbling around the edges.
Carefully remove the foil (watch for steam), then return the uncovered dish to the oven. Bake another 10–15 minutes, or until the cheese is fully melted, lightly browned in spots, and the peppers are tender when pierced with a fork.
Let the casserole rest on a heatproof surface for about 5–10 minutes before serving. This helps the juices settle a bit so you get nice, hearty scoops of beef, peppers, and gooey cheese, just like in that glass baking dish that stole the show at Easter.
