Preheat broiler. For dressing, in a small bowl mash eight of the berries. Stir in mustard, vinegar, and sugar. Whisk in olive oil; set aside.
Lightly sprinkle the steak with salt and pepper, then lightly brush with additional olive oil. Heat a grill pan over medium-high heat. Cook steak 12 to 14 minutes, turning once. Slice steak into strips.
Meanwhile, generously grease a 13x9x2-inch baking pan. Toss together potatoes and cheese; spread in pan. Broil 3 to 4 inches from the heat for 6 to 8 minutes or until golden brown and crisp. Remove from pan; cut in pieces.
Remove core from lettuce; cut crosswise into four pieces. Serve with steak, potatoes, and berries. Pass dressing.