Preheat oven to 400°.
Mix garlic, coriander, cumin, and 3 Tbsp. oil in a large bowl.
Add cauliflower and chickpeas; season with salt and pepper. Toss to coat.
Transfer cauliflower mixture to a parchment-lined rimmed baking sheet. Wipe out bowl; reserve. Bake, tossing halfway through, until chickpeas and cauliflower are golden brown and cauliflower is tender, 18–22 minutes.
Meanwhile, mix yogurt, 1 Tbsp. lemon juice, and a big pinch of salt in a small bowl.
Toss fennel, chile (if using), parsley, sesame seeds, and remaining 3 Tbsp. lemon juice in reserved bowl; season with salt.
Spread yogurt mixture on slices of toast. Top with spiced chickpea mixture. Scatter fennel salad over, then drizzle with oil.