Preheat the oven to 400 degrees F (200 degrees C).
Combine ground beef, sausage, egg, breadcrumbs, milk, Italian seasoning, salt, and pepper in a large bowl. Roll meat mixture into 1-inch meatballs and place onto a baking sheet.
Bake meatballs in the preheated oven until cooked through, 15 to 20 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (71 degrees C).
Meanwhile melt butter in a large skillet over medium heat. Add olive oil and onion; continue cooking until softened, about 5 minutes. Add garlic, red pepper flakes, salt, pepper and tomato paste; stir continuously until fragrant, about 1 minute.
Pour in water and bring to a boil. Stir in pasta, reduce heat, and cover with a lid. Continue cooking until pasta is al dente, stirring occasionally, 10 to 15 minutes.
Before serving, stir in meatballs and cheese.
