In a coffee grinder or with a mortar and pestle, coarsely grind the flaxseeds, fennel, mustard seeds and peppercorns. Pour into a plate and add the sesame seeds. Stir to combine. Press the tuna steaks into the spice mixture to coat. Season with salt.
In a large non-stick skillet, brown the tuna steaks in the oil for 2 to 3 minutes per side or to the desired doneness. Season with salt and pepper. Thinly slice and drizzle with olive oil, if desired. Serve with Chickpea, Apple, and Zucchini Salad (see recipe below).