Preheat the grill to medium-high heat.
In a large bowl, toss the sliced red onion, cherry tomatoes, quartered mushrooms, and sliced zucchini with olive oil, salt, and pepper until evenly coated.
Place the seasoned vegetables on the preheated grill and cook for 8-10 minutes, flipping occasionally, until they are tender and lightly charred.
While the vegetables are grilling, prepare the shrimp. In a separate bowl, toss the shrimp with olive oil, minced garlic, lemon zest, salt, and pepper.
Once the vegetables are done grilling, remove them from the grill and set aside. Place the seasoned shrimp on the grill and cook for 2-3 minutes per side, or until they are pink and opaque.
In a small bowl, whisk together the lemon juice, olive oil, Dijon mustard, honey, salt, and pepper to make the lemon vinaigrette.
In a large serving bowl, combine the grilled vegetables, grilled shrimp, and mixed greens. Drizzle the lemon vinaigrette over the salad and toss gently to coat.
Serve the spring mix salad with grilled shrimp and vegetables immediately, garnished with additional lemon zest if desired.