While preheating the oven to 350°F, cut the cauliflower into pieces and arrange on ⅓ of the pan. Place in the oven and bake for about 15 minutes.
Meanwhile, marinate the shrimp with pesto in a bowl. Let sit for a few minutes.
Take out the pan from the oven. Arrange the tomatoes in the middle and the lemon slices in the other side.
Place the shrimps over the lemon slices. Sprinkle the olive oil over the tomatoes and cauliflower. Add the garlic powder, lemon juice (about 2 tbsp), and onion to the pan. Put some dash of salt and pepper to taste.
Replace in the oven and bake for 15-20 minutes more. Serve.
If you want to lower net carb, consider using less tomatoes.