Bring stock to a simmer in a medium pot over medium heat.
Heat butter in saucepan. Add onion and garlic and cook until softened about 4–5 minutes.
Stir in rice; season with salt and pepper. Cook, stirring often, until grains turn translucent around edges, about 3 minutes. Add wine, cook until pan is almost dry. Ladle in 1 1/2 cups warm stock, bring to a simmer, and cook until liquid is absorbed. Ladle in 1 cup stock, continue to cook until most liquid is absorbed. Add remaining 1½ cups stock, cook until rice is al dente and liquid is mostly absorbed. Remove from heat. Stir in Parmesan and cream.
Spread risotto evenly over parchment-lined rimmed baking sheet. Cover with plastic wrap and chill for 1 hour.
Scoop risotto into your hand and form into a patty. Place 1 piece mozzarella and 1 piece tomato in center of patty. Carefully pinch and shape risotto so it completely encases the cheese, then roll into a 2" ball. Place on prepared baking sheet. Repeat with remaining risotto and mozzarella. Place balls in freezer 10 minutes.
Place flour in 1st bowl, eggs in 2nd bowl, and breadcrumbs in third bowl, season all bowls with salt and pepper. Dredge and coat balls in flour, then egg, and then breadcrumbs until all rice balls have been breaded. Refrigerate until needed.