How to Make It
Cut a slice off the stem end of each tomato. Using a spoon, hollow out the tomatoes. Season with salt and piment d'Espelette. Cook all the haricots verts, then the carrots in a medium saucepan of boiling salted water until just tender, 3 to 5 minutes for each. Drain, rinse in cold water and drain on paper towels.
In a medium bowl, combine 1 tablespoon of lemon juice with the mustard. Season with salt and piment d'Espelette, then whisk in the crème fraîche. Stir in the haricots verts, carrots, spring onions and chopped chives. Fill the tomatoes with the halved haricots verts and some of the spring onions and carrots.
Heat a large skillet. Brush the tuna steaks with the olive oil and the remaining 1 tablespoon of lemon juice and season with salt and crushed black peppercorns. Add the tuna to the skillet and sear over high heat until well browned, about 2 minutes per side.
Mound the remaining vegetables on one side of each plate. Lean the tuna steaks off center on the vegetables and top with the greens. Set a stuffed tomato alongside each. Dust the plates with piment d'Espelette, garnish with chives and lemon wedges and serve.