Layer onions at the bottom of the instant pot insert. Sprinkle with garlic slices. Pour crushed tomatoes over onions and garlic.
Layer vegetables vertically over sauce, filling the dish. Drizzle vegetables with olive oil. Sprinkle with salt, pepper, and italian seasoning.
Seal Instant Pot. Set vent to sealed. Program for "manual" and "high pressure" and set timer for 5 minutes.
After it's done, allow natural release (takes 10-15 minutes). Open instant pot, layer fresh mozzarella over vegetables, and sprinkle with fresh basil ribbons.