Heat grill pan. Rub tuna fillets with 1 TBSP olive oil and 1/4 tsp each salt and pepper; sear 1-2 minutes per side. In skillet, cook shallot in 2 tsp olive oil until soft. Add peppercorns, vinegar, and thyme; reduce by half. Add orange juice and 2 TBSP lemon juice. Whisk in butter. Strain; season with 1/4 tsp salt. In clean skillet, sauté garlic in 1 TBSP olive oil. Add arugula, 2 tsp lemon juice, and 1/4 tsp each salt and pepper; let wilt.