In a large nonstick or well-seasoned cast-iron skillet over medium-high heat, combine the zucchini and onion with olive oil, the red-pepper flakes and 1 teaspoon salt. Cook the mixture, stirring once or twice, until the zucchini and onion turn golden brown, 10 to 12 minutes. Don’t stir too often, as it can impede browning.
Stir in stock and bring to a simmer. Stir in orzo, lemon zest and ½ teaspoon salt. Cover and simmer over medium-low heat until orzo is nearly cooked through and most of the liquid is absorbed, 10 to 14 minutes, stirring once or twice.
Meanwhile, in a small bowl, toss together the tomatoes, mozzarella, a pinch of salt, a pinch of red-pepper flakes and a drizzle of olive oil, and let marinate while the orzo cooks.
Once the orzo is ready, stir in juice of ½ lemon, Parmesan, mint and pesto. Cover the pan, and cook for 1 minute, to finish cooking. Taste for seasoning and add more lemon juice or pesto, if needed. To serve, top with tomato-mozzarella mixture and sprinkle with more cheese and mint.
