In a saucepan, melt butter over medium heat.
Stir in shallot, salt and pepper, and sauté until lightly browned.
Add wine and simmer until reduced by half, then stir in cream and mustard and simmer until sauce is reduced to approximately 1 cup.
Stir in fresh chopped tarragon, lemon juice, and salt to taste. Set aside.
Brush tuna loins with olive oil and season all sides with salt and pepper.
Heat grill to high heat. Arrange tuna loins directly on grill grate, and cook about 3 minutes per side, or to the desired doneness. Fish should stay rare in the center.
Transfer loins to cutting board and slice to 1” thick medallions. Serve with sauce spooned over top, and garnish with fresh tarragon and lemon wedges.