Fill a large pot with about 3 quarts of water and plenty of salt (about 2 tablespoons). Bring to a boil, then cook your pasta according to the instructions on the box. Reserve 1 cup pasta water, then drain and set aside.
In a large skillet over medium-high heat, melt the butter and olive oil. Stir in the zucchini, fennel, garlic, ½ teaspoon salt, and pepper. Spread into an even layer and cook until the water has evaporated and the mixture is “jammy” in texture and begins to stick to the pan, about 10-15 minutes, stirring occasionally.
If using spinach, add it in with the remaining ½ teaspoon salt. Cook until the spinach wilts down, about 3 minutes, stirring often.
If you aren’t using spinach, add the ½ teaspoon salt in with the pasta below.
Add in the pasta and ½ cup reserved pasta water. Simmer and toss the pasta until the liquid reduces and forms a glossy sauce, about 2-3 minutes. Remove from heat and stir in the parmesan and lemon. If the pasta is looking dry, just add in a splash more pasta water. Taste and season with salt, pepper, or more lemon, if needed.
Serve topped with freshly grated black pepper.