Preheat the oven to 475°F (245°C).
Lightly spray a rimmed sheet pan with olive oil.
Arrange the zucchini slices and diced onion in an even layer on the pan.
Spray lightly with olive oil and season with kosher salt.
Roast on the lower oven rack for 8 minutes, rotating the pan halfway through cooking.
While the vegetables roast, place the shrimp in a bowl.
Add olive oil, lemon zest, grated garlic, kosher salt, and black pepper.
Toss until the shrimp are evenly coated.
Remove the sheet pan from the oven.
Flip the zucchini slices and push them toward the outer edges of the pan.
Arrange the shrimp in a single layer in the center.
Return the pan to the oven and roast for 6–8 minutes, or until the shrimp are opaque and fully cooked.
Remove from the oven and squeeze fresh lemon juice over everything.
Sprinkle with toasted pine nuts, chopped parsley, and red pepper flakes if desired.
Serve immediately with cauliflower rice, brown rice, or white rice.