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.