Preheat oven to 180ºC. Season the steaks with salt and pepper, then rub with half the olive oil. Combine the butter, lemon zest and thyme in a bowl and beat well. Toss the tomatoes in the remaining olive oil and season with salt.
Cook the steaks over a hot barbecue grill for 8 minutes turning regularly, until medium. Set aside to rest for 3 minutes. Meanwhile, put the tomatoes in an oven tray and bake for 5 minutes, until just hot to touch. Toss with the onion, basil and lemon juice. Spoon the butter over the steaks and serve with the tomatoes and broccolini.