Halve and peel onion. Slice half the onion into thin strips and cut other half into 1/4" dice. Combine lemon-herb seasoning and smoked paprika in a mixing bowl with 2 Tbsp. olive oil. Add cauliflower florets and diced onion (reserve 2 tsp. for lemon vinaigrette). Toss to coat and spread into a single layer on prepared baking sheet. Roast until tender and lightly caramelized, 18-20 minutes. Reserve mixing bowl for dressing vegetables; no need to wipe clean.

Trim cucumbers. Halve one lengthwise and cut into 1/4" half-moons. Grate second cucumber on large hole side of box grater and squeeze out excess moisture in a paper towel. Core Roma tomato and halve lengthwise. Slice one half into 1/4" half-moons and finely dice other half. Zest lemon, halve, and juice.

In a small bowl, combine yogurt, grated cucumber, 1 tsp. lemon juice, 1 tsp. lemon zest, 1/2 tsp. salt, and 1/4 tsp. pepper. In a second small bowl, whisk together 1 Tbsp. lemon juice, reserved 2 tsp. diced red onion, 2 Tbsp. olive oil, 1/4 tsp. salt, and a pinch of pepper.

Stack pitas and wrap in foil. Place in oven 5 minutes until warmed and pliable. Combine diced tomato and half the half-moon cucumbers (reserve remaining for spinach salad) in reserved mixing bowl. Toss with 1 tsp. lemon vinaigrette.

Fill warmed pitas with cauliflower and dressed vegetable mixture. In mixing bowl used for vegetables, combine spinach with thin onion strips (to taste), half-moon tomatoes, and remaining half-moon cucumber. Toss with remaining lemon vinaigrette and season to taste with a pinch of salt and pepper.

Serve pitas on a plate along with salad. Garnish salad with remaining lemon zest. Add a dollop of tzatziki sauce to pitas or serve on side for dipping.