A great summer salad requires three things: fresh greens, a tart, seasonal dressing (made here with grated tomato), and a light hand to toss the ingredients together so the lettuces stay perky.
Season steak with 1 teaspoon 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 teaspoon 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″ 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 teaspoons 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″ 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.
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