Using a stainless steel pan heated very hot (you can feel the heat when you hold your hand 6" above the pan) put in 1 tsp oil and roll it around. Then place in your seasoned tuna steak. It needs to sizzle and crust - this should take about 3 minutes.
Remove from heat and flip the tuna steak. Do NOT put it back on the heat.
Put a pat of butter and some lemon juice in the pan along with small diced a shallots, stirring until the shallots are translucent.
Serve immediately with drizzle of the juice.
What you're looking for is a slightly pink center for well done to a red strip through the middle for rare.