1) Season both sides of tuna with salt and black pepper. In a large skillet, heat oil over medium-high heat. Add tuna to hot pan and cook for 2 minutes per side, until golden brown.
2) In a medium bowl, combine stock, tomatoes, shallots and tarragon. Add mixture to skillet and bring to a simmer. Simmer 2 minutes, until tuna is cooked to medium (or longer for more well done fish).
3) Finish the sauce with butter.
Serve four tuna fillets with all of the sauce and rice. Reserve the remaining tuna for melts, if desired.