Preheat the oven to 350 degrees F (175 degrees C).
Heat a cast iron skillet over medium-high heat. Spray pork chops with cooking spray and season with garlic powder, paprika, and chili powder. Brown chops in the hot skillet, turning frequently, until lightly browned on both sides, 5 to 8 minutes total. Transfer to a casserole or baking dish.
Mix mushroom soup and diced tomatoes together in a large bowl. Pour over pork chops and place onion slices on top. Cover the pan tightly with aluminum foil.
Bake in the preheated oven for 40 minutes. Remove foil and continue to bake until pork is tender, 5 to 10 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).
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