Season the tuna steaks with salt and pepper and saute or broil, whichever you prefer. (Tuna is at its best when cooked rare or medium rare.)
Set aside and keep warm in a 140F oven.
In a large saute pan, heat the olive oil, add the garlic and shallots, and stir for 30 seconds; add the tomatoes and stir for 30 seconds.
Add the spinach and vinegar; toss until limp but completely cooked.
Remove from the heat and toss in the feta cheese.
Notes
Spread on a serving dish and place tuna steaks on top. Garnish by sprinkling with a mixture of diced tomatoes, feta cheese, and chopped parsley and lemon wedges.