Fennel and Coriander Crusted Tuna with Aioli

Best new recipe of the summer. Proportions of spices came from Bon Appetit, as I had a free subscription this summer. But rest of recipe was a wreck - trying for a one-pan meal with vegetables and couscous. Plus 1 1/2" tuna could not have been rare if cooked for 4 minutes a side. That's 1 1/2" ribeye cooking time, not tuna.

2 teaspoons fennel seed
1 teaspoon coriander seed
1 teaspoon black peppercorns
1 teaspoons kosher salt

3 T mayonnaise
Zest of one lemon
1 tablespoon fresh lemon juice
1 tablespoons fresh chives
2 garlic cloves

1 1/2 pounds tuna steaks (1 inch thick or more)
Olive oil

1. Prepare grill.
2. Toast fennel, coriander and pepper over medium heat, about 3 minutes. Grind spices and add salt.
3. Whisk mayonnaise, lemon juice, chives, and half of garlic in small bowl. (Can be made 2 days ahead. Store spice mixture airtight at room temperature. Cover and refrigerate lemon aioli.)
4. Coat tuna with olive oil. Sprinkle with spice mixture, press down. Sear tuna until. Rare center is about 2 minutes per side. Optional: refrigerate 1 hour, then cut into slices.
5. Serve with aioli.