Preheat the oven to 375 degrees F.
In a medium bowl, combine the sausage, baking mix, and cheese. Mix very well with your hands, or in a food processor, until the meat and cheese are evenly distributed (the mixture will seem dry). Roll and press the mixture into 1-inch balls.
Transfer the sausage balls to a baking sheet and flatten each one with the palm of your hand. Make a small indentation in each patty with your thumb and nestle a tomato half, cut side up, in the indentation.
Bake until the meat is cooked through, 15 to 25 minutes. Broil for 3 minutes. Serve hot or warm.