Step 1
Build a medium fire in a charcoal grill, or heat a gas grill to medium-high. Lightly sprinkle salt over cut sides of eggplant and let sit for 15 minutes (you will see beads of water form on the surface). Rinse eggplant and pat dry.
Step 2
Lay eggplant, tomatoes, squash, bell pepper, and scallions on a baking sheet and drizzle with 1 Tbsp. oil; season with salt and pepper.
Step 3
Grill vegetables, turning occasionally, until tender, transferring them to a baking sheet as they are done. The tomatoes, scallions, and eggplant will cook the fastest (2–5 minutes); the squash and bell pepper will take about 5 minutes.
Step 4
Slice three-quarters of the tomatoes in half and grate the cut side over the coarse holes of a box grater into a medium bowl (discard core and any tomato skin left in your hand). Whisk in vinegar and remaining 2 Tbsp. oil; season to taste with salt and pepper. Set vinaigrette aside.
Step 5
Chop remaining tomatoes and transfer to a large bowl. Chop remaining grilled vegetables into 1/2-inch pieces and add to bowl. Drizzle tomato vinaigrette over and toss gently to coat. Transfer vegetables to a serving platter or bowl and garnish with tarragon.
