How to Make It
Whisk together vinegar, oil, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper in a small bowl. Reserve 2 tablespoons vinegar mixture in a separate small bowl.
Heat a grill pan over medium-high. Brush salmon fillets evenly with reserved 2 tablespoons vinegar mixture. Add salmon, skin side down, to grill pan; cook 5 minutes. Flip salmon, and cook 2 to 3 minutes or to desired degree of doneness. Transfer salmon to a plate; remove and discard skin.
Add bread slices to grill pan; cook until toasted and grill marks appear, 2 to 3 minutes per side. Transfer bread to a cutting board; cut into 1-inch cubes.
Stir together bread cubes, tomatoes, and remaining vinegar mixture, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a bowl until combined. Break salmon into bite-size pieces. Gently fold salmon and spinach into bread mixture. Serve at room temperature.