How to Make It
Preheat the broiler. Brush the tuna steaks on both sides with olive oil and season with salt and pepper. Broil half the tuna steaks for 2 minutes per side, until just rare. Repeat with the remaining tuna steaks. Let the tuna cool to room temperature, then finely chop it with a knife.
In a large bowl, combine the tuna with the cornichons, red onion, dill, capers and olives. Fold in 1 cup of the mayonnaise and the lemon juice and season with salt and pepper. Form the tuna into 10 burgers and transfer to a baking sheet. Refrigerate until firm, about 30 minutes.
In a small bowl, whisk the remaining 1 cup of mayonnaise with the Dijon and dry mustards. Season with salt and pepper. Refrigerate the mustard mayonnaise until it is ready to use.
Dust the tuna burgers with flour. Heat 1/4 inch of olive oil in each of 2 large skillets until shimmering. Add 3 or 4 burgers to each skillet and cook over moderately high heat until browned and crisp, about 3 minutes per side for medium. Drain the tuna burgers on paper towels and cook the remaining burgers in 1 skillet. Serve the tuna burgers with the mustard mayonnaise. Pass the Marinated Cabbage Slaw separately.