A delicious Dijon vinaigrette brings all the ingredients together in this simple salad.
Preheat oven to 450 degrees. On a large rimmed baking sheet, toss bread with 2 tablespoons oil; season with salt and pepper. Bake, tossing occasionally, until toasted, 4 to 6 minutes. (To store, keep croutons in a resealable bag, up to 1 week.)
In a large bowl, whisk together remaining oil, vinegar, and Dijon; season with salt and pepper. (To store, refrigerate dressing, up to 1 week.)
Toss mushrooms with dressing; let stand 5 minutes. Add spinach, onion, and croutons to bowl; toss, and serve immediately.
Making your own croutons is economical and easy. Toss cubed bread with a little olive oil, season, and bake until toasted.