Grilled Steak Salad with Tomato Vinaigrette

1 pound hanger, skirt, or flank steak

1 teaspoon kosher salt plus more

Freshly ground black pepper

1 medium tomato (about 6 ounces), halved

1 tablespoon minced shallot

1 teaspoon red wine vinegar

1/4 cup plus 5 tsp. olive oil, divided

4 spring onions or 6 scallions, bulbs halved, dark-green parts discarded

5 1/2-inch-thick slices ciabatta

8 cups mixed summer lettuces (such as mizuna, baby mustard greens, and tatsoi)

3/4 cup fresh basil leaves, torn into 1/2-inch strips

Season steak with 1 tsp. salt and pepper; set aside. Grate cut sides of tomato on coarse holes of a box grater into a medium bowl down to the skin; discard skin. Add shallot and vinegar; whisk in 1/4 cup oil. Season to taste with salt and pepper. Set aside. DO AHEAD: Tomato vinaigrette can be made 1 day ahead.

Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Toss spring onions in a medium bowl with 1 tsp. oil and season with salt and pepper. Grill onions until just tender, 2–3 minutes per side. Transfer to a cutting board and cut into 2-inch pieces.

Grill steak until seared and cooked to desired doneness, 3–5 minutes per side for medium-rare, depending on steak's thickness. Transfer to a cutting board. Let rest, about 10 minutes.

Meanwhile, make croutons: Brush both sides of bread slices with remaining 4 tsp. oil and season with salt and pepper. Grill bread until dark golden brown and nicely charred in spots, about 2 minutes per side. Set toast aside until cool enough to handle, then break toast into roughly 1-inch pieces.

Thinly slice steak against the grain. Toss lettuces, basil, spring onions, croutons, and some of the vinaigrette in a large bowl. Season to taste with salt and pepper. Add steak and toss gently to coat. Serve with remaining vinaigrette alongside.

Serves 4-6.