Spinach salad with mushrooms and blue cheese

3 tablespoons red-wine vinegar
1 tablespoon olive oil
salt, pepper
5 ounces baby spinach (about 5 cups)
8 ounces white button mushrooms, stems trimmed, thinly sliced
small red onion, thinly sliced
½  red bell pepper, ribs and seeds removed, diced
2 ounces crumbled blue cheese (scant 1/2 cup)

In a large bowl, whisk together vinegar and oil; season with salt and pepper.

Add spinach, mushrooms, onion, bell pepper, and blue cheese to bowl; toss to coat with
dressing. Serve immediately.