Preheat a grill pan on high heat for 5 minutes. Meanwhile, rub the beef all over with the garlic clove, coat with olive oil and season on all sides with salt and pepper. Reduce the heat to medium high and grill the steak over direct heat covered with a metal bowl, turning once, until medium rare, about 10 minutes. Transfer to a cutting board and let rest for 5 minutes. Cut into 1/2-inch-thick slices. Cut the slices into 1-inch squares.
Combine the cheese, basil, lemon juice, sugar and a pinch of salt and pepper in a small bowl; mix until thoroughly combined. Spoon the mixture into a resealable plastic bag with 1 corner snipped.
To assemble, pat dry the cucumber slices. Pipe a small dollop of the cheese mixture onto each cucumber round. Place a square of steak on top, then pipe a teaspoon of the cheese mixture on top. Garnish each with the tomatoes and onion.