Preheat oven to 400 degrees F. Line a baking sheet with parchment or foil.

Place chicken on the prepared baking sheet. Sprinkle all over with salt, thyme, and pepper. Roast, flipping once, until the chicken is cooked through and reaches an internal temperature of 165°F, about 15 to 17 minutes. Set aside to cool, then slice into bite-size pieces.

Divide spinach among 4 single-serving lidded containers (2 cups each). Top each with one-fourth of the sliced chicken, 1/2 cup sliced strawberries, 1 tablespoon feta (if using), and 1 tablespoon walnuts.

Seal the salad containers and refrigerate for up to 4 days.

Transfer 1 1/2 tablespoons vinaigrette into each of 4 small lidded containers and refrigerate for up to 5 days.

Dress the salads with the vinaigrette just before serving.