Heat the oven to 400°F. Place the farro in an 9×9 inch or 11×7 inch glass/ceramic baking dish (depending on how deep the dish is) and transfer to the oven to toast, 10 to 15 minutes. Remove the dish from the oven and scatter the lentils over the farro.

While the farro toasts, make the tomato sauce: In a medium saucepan over medium-high heat, heat 2 tablespoons olive oil. Add the onion, season with salt and pepper, and cook, stirring occasionally, until softened, 5 to 7 minutes. Add the garlic and cook, 1 to 2 minutes. Add the tomatoes and juice, herb sprigs (if using), grated zucchini and 2 ½ cups water.

Gently pour the tomato sauce over the lentils and farro and stir to combine. Bake, stirring halfway through, until the lentils and farro are tender, about 40 to 50 minutes.

Add 3/4 cup water, stir, then top with grated mozzarella and bake an additional 15 minutes or until the cheese is melted and starting to turn golden.

Remove the herb sprigs as you serve.