DO-AHEAD TIP: Cook collard greens.
Preheat oven to 375 degrees.
Melt the coconut oil in a large skillet over high heat. Sear each tuna steak for 2 to 3 minutes per side.
Layer sliced onion and tuna steaks in a large baking dish; set aside.
In a small bowl, whisk together next 10 ingredients (salt and pepper through sesame seeds); pour mixture over tuna and onion. Bake for 10 to 15 minutes or until onion is slightly tender and each tuna steak has a thick strip of pink in the center.
Serve tuna and onion over a bed of collard greens.