Heat the olive oil in a large saucepan or Dutch oven over medium heat. Add the diced onion and cook, stirring occasionally, for about 4 minutes until it softens and turns translucent. Do not rush this step. Soft onion gives the broth a sweeter base.

Add the minced garlic and red pepper flakes if using. Stir and cook for about 1 minute until the garlic smells fragrant but has not browned. Garlic burns quickly at this stage, so keep it moving.

Add the diced zucchini, season with the salt and black pepper, and stir to coat everything in the oil. Let the zucchini cook for 2 minutes, just long enough to start softening slightly on the edges.

Pour in the chicken broth and drop in the Parmesan rind. Raise the heat to medium-high and bring the broth to a gentle boil. You will see the rind start to soften and release its flavor into the liquid.

Stir in the dry orzo. Reduce the heat to a steady simmer and cook uncovered for 9 to 10 minutes, stirring every couple of minutes so the orzo does not stick to the bottom. The orzo should be tender but still have a slight chew, and the broth will thicken slightly from the starch.

Fish out the Parmesan rind with a spoon and discard it. Taste the broth and adjust salt and pepper as needed. Ladle the soup into bowls, top each one with a generous pinch of grated Parmesan, a drizzle of olive oil, and a little chopped parsley if you have it. Serve right away with crusty bread on the side.