For the Meatballs:

• 1 small onion, grated
• 1 1/2 pounds extra lean ground beef
• 1/2 green bell pepper, finely chopped
• 1/2 red bell pepper, finely chopped
• 2 cloves garlic, minced
• 2 tablespoons Worcestershire sauce
• 2 large eggs
• 1/2 cup grated Parmesan cheese
• 1/2 cup shredded Gruyère cheese
• 2 tablespoons olive oil, for frying

For the Cheese Sauce:

• 2 tablespoons unsalted butter
• 1 tablespoon all-purpose flour
• 1 cup milk (3% or your preferred type)
• 1 cup shredded Gruyère cheese
• 1 cup shredded cheddar cheese
• 1/4 teaspoon salt, or to taste
• 1/4 teaspoon black pepper, or to taste
• Fresh parsley, chopped (optional, for garnish)