Preheat the oven to 400 degrees F (200 degrees C).
Open the "pocket" on the wide end of the leg of lamb and stuff with some garlic slices, 1 rosemary sprig, 1 bay leaf, and salt. Make 3 long incisions on the top of the leg with your knife. Stuff remaining garlic, rosemary sprigs, bay leaves, and some salt into the cuts. Season well with salt all over.
Season eggplant, zucchini, and 1 red onion with salt and tumble into a roasting pan. Place the leg of lamb on top.
Roast in the preheated oven for 20 minutes. Reduce oven temperature to 300 degrees F (150 degrees C) and cover pan with aluminum foil. Continue baking until lamb is tender, about 2 hours 30 minutes.
Spread potatoes on a baking sheet. Drizzle with 2 tablespoons olive oil and season with sea salt. Place in the bottom third of the oven. Bake lamb and potatoes for 30 minutes. Remove lamb and let rest while potatoes finish cooking, about 30 minutes more.
Combine grated cucumber, mint, oregano, and salt in a bowl. Mix in yogurt until well combined. Refrigerate until ready to serve.
Combine tomatoes, sliced cucumber, sliced red onion, and black olives in a deep-sided dish. Drizzle 3 tablespoons olive oil and red wine vinegar on top. Place feta on top and cover with dried oregano.
Carve lamb and serve on a big platter. Scoop out vegetables and juices from the roasting pan and add to the platter. Place roasted potatoes, tzatziki, and Greek salad nearby.