Preheat the oven to 475°F (245°C).
Spray a rimmed baking sheet with olive oil.
Arrange the zucchini slices and diced onion in an even layer on the baking sheet.
Spray lightly with additional olive oil and season with a few pinches of kosher salt.
Roast on the bottom rack for 8 minutes, rotating the pan halfway through cooking.
While the vegetables roast, place the shrimp in a bowl.
Toss the shrimp with olive oil, lemon zest, grated garlic, kosher salt, and black pepper.
Remove the baking sheet from the oven and carefully flip the zucchini slices.
Push the vegetables toward the edges of the pan and arrange the shrimp in a single layer in the center.
Return the pan to the oven and roast for 6 to 8 minutes, or until the shrimp are opaque and fully cooked.
Remove from the oven and squeeze fresh lemon juice over the top.
Sprinkle with toasted pine nuts, chopped parsley, and red pepper flakes if desired.
Serve your Sheet Pan Lemon Garlic Shrimp and Zucchini with cauliflower rice, brown rice, or white rice.