Heat a large Dutch oven or soup pot over medium-high heat, then cook the ground beef and onion for about 5 minutes, crumbling the meat as it browns; add 1 tablespoon olive oil only if very lean beef needs help getting started. The beef should lose its raw color and pick up browned bits on the bottom of the pot while the onion softens.

Stir in the garlic and cook for 1 minute, stirring constantly. Once the garlic smells sweet and fragrant, move on before it darkens.

Pour in the chicken broth, spaghetti sauce or marinara sauce, petite diced tomatoes, parsley, Italian seasoning, salt, black pepper, and red pepper flakes, then stir and bring the pot to a boil. The broth should look evenly combined and deeply red, with little bubbles breaking across the surface.

Add the broken lasagna noodles and cook for 11 to 13 minutes, or according to the package, until al dente. The noodles should bend easily but still show a slightly firm center, and the soup should lightly coat the back of a wooden spoon as the starch releases.

Turn off the heat, add the fresh spinach, and stir until wilted, about 1 to 2 minutes. The spinach should soften into the soup quickly while staying green and tender.

Taste and adjust the seasoning as needed, then add the granulated sugar if you want a softer tomato edge. The finished broth should taste balanced, savory, and rich rather than harsh or flat.

Mix the ricotta cheese, mozzarella cheese, Parmesan cheese, Italian seasoning, and optional salt in a medium bowl until evenly combined. The mixture should look thick, creamy, and evenly speckled with seasoning.

Ladle the hot soup into bowls, add a dollop of the cheese mixture, finish with fresh herbs if desired, and serve right away. The cheese should soften at the edges and melt slightly into the hot broth.