Heat your oven to 350ºF.
Heat the oil in a large, oven-proof skillet over medium-high heat. Add the eggplant, zucchini, leek, and fennel; cook 10 minutes, or until soft. Add the garlic, tomatoes, and thyme. Season with salt and pepper.
In a small bowl, combine the flour, oats, parmesan, and lemon zest. Stir in the olive oil until everything is combined and all of the dry ingredients are moistened. Sprinkle over the vegetables in an even layer.
Transfer the pan to the oven and bake for 30 minutes, or until the top of the crumble is browned and crisp.
Serve with additional parmesan, as desired.