Toss broccoli with 1 tablespoon olive oil, spread on a sheet pan, and roast at 425°F for 12 minutes.
While broccoli roasts, mix butter with wasabi paste and soy sauce in a small bowl.
Pat tuna steaks dry, brush with remaining olive oil, and sear in a hot skillet over medium-high heat for 2 minutes per side (125°F internal temperature for medium-rare); transfer to plates and top each with a dollop of wasabi butter.
Toss roasted broccoli with sesame seeds and lemon juice, divide among plates, and serve immediately.