Preheat oven to 400°F (200°C). Spray a sheet pan with nonstick spray or line it with foil.
If not already halved, slice the mini rainbow peppers in half lengthwise and remove the seeds. Arrange the pepper halves cut-side up in a single layer on the prepared sheet pan. Set the pan aside.
Heat a large skillet over medium heat. Add the 1 lb 90% ground beef and cook, breaking it up with a spoon, until browned and no pink remains (about 6–8 minutes). Drain any excess liquid or fat from the skillet.
Return the skillet to medium heat. Add the 2 cloves minced garlic, 2 tsp chili powder, 1½ tsp cumin, 1 tsp dried oregano, ½ tsp onion powder, and ½ tsp salt. Stir and cook about 30–60 seconds, until the garlic is fragrant and the spices coat the meat.
Stir in ½ cup chunky salsa and cook 2–3 minutes more, until the mixture is heated through and slightly thickened.
Spoon the meat mixture evenly into each pepper half, filling them to the top. Sprinkle 1 cup shredded Colby Jack or Mexican cheese evenly over the filled peppers.
Bake the sheet pan in the preheated oven for 10–12 minutes, until the peppers are tender and the cheese is melted.
Remove the pan from the oven and top the baked peppers with ½ cup chopped cherry tomatoes, ¼ cup sliced black olives, 1 or 2 sliced jalapeños, and 1 to 2 Tbsp chopped fresh cilantro. Add sour cream, avocado, or guacamole if desired, and serve warm.