Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
Put the cornbread stuffing cubes in a large bowl and gently crush them into smaller pieces. Add the ground beef, black beans, egg, onion, chili powder, cumin, salt, and 1/4 cup of the salsa.
Mix gently with your hands or a fork just until everything is combined. Avoid packing the mixture tightly, which can make the muffins dense.
Divide the mixture among the 8 prepared muffin cups, pressing lightly to shape each portion. Spoon the remaining 1/4 cup salsa over the tops.
Bake for 20 minutes. Sprinkle the shredded Monterey Jack over the muffins, then bake for 5 to 10 minutes more, until the centers reach 160°F.
Let the muffins rest in the pan for 5 minutes before removing and serving.