Preheat a well-oiled grill to medium-high heat. (TIP: If using a grill pan, you’ll heat it in Step 2.) Wash and dry produce.

Dice potatoes into ¾-inch pieces. Place in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until fork-tender, 10-12 minutes. Drain and rinse under cold water; transfer to a large bowl and let cool completely. TIP: Spread potatoes out around edges of the bowl to help them cool more quickly.

Place 2 TBSP butter (4 TBSP for 4 servings) in a small microwave-safe bowl. Microwave until just softened, 10-15 seconds.

Stir in half the parsley and a pinch of lemon zest (big pinch of zest for 4) until combined. Set aside.

Pat steak* dry with paper towels. Season all over with garlic powder, salt, and pepper.

Add steak to grill (or grill pan, working in batches for 4 servings if needed); grill to desired doneness, 4-8 minutes per side (grills can vary).

Transfer steak to a cutting board to rest for at least 5 minutes.

To bowl with cooled potatoes, add mayonnaise, half the mustard, remaining parsley, and juice from one lemon wedge (two wedges for 4 servings); stir to combine. Season with salt and pepper. Cover with plastic wrap and refrigerate until ready to serve.

In a second large bowl, whisk together remaining mustard, 3 TBSP olive oil, and juice from two lemon wedges (6 TBSP olive oil and four wedges for 4); season with salt and pepper. Add lettuce and tomatoes to bowl; toss to coat.

Slice steak against the grain.