Put the lemon juice and capers in a 1-qt. heatproof measuring cup. Bring the butter and cream to a brisk simmer in a small saucepan over medium heat, whisking often. The mixture will look homogenous at first and then will separate and begin to turn golden brown. Continue to whisk until it turns a dark rust color, about 10 minutes total. Carefully pour the brown butter mixture into the lemon juice; the butter will boil up and sputter. Whisk to combine and return to the saucepan, off the heat.
Sprinkle the tuna on both sides with the fennel seeds, salt, and pepper. Heat the oil in a 10- to 12-inch heavy skillet (preferably cast iron) over medium-high heat until shimmering hot. Sear the tuna on both sides until done to your liking, about 2 minutes per side for medium rare (still raw in the middle) or 3 minutes per side for medium (just pink in the middle). Transfer the tuna to plates. If necessary, quickly reheat the sauce. Serve the tuna drizzled with the sauce.
This tuna is delicious sprinkled with fresh parsley, and it pairs well with roasted fennel.
nutrition information (per serving):
based on four servings;
Photo: Scott Phillips