1 1/2 tablespoons extra-virgin olive oil |
1 tablespoon white wine vinegar |
1/2 teaspoon dijon mustard |
1/4 teaspoon freshly ground black pepper |
1/8 teaspoon salt |
1/4 cup vertically sliced red onion |
3 garlic cloves, thinly sliced |
1 (9-ounce) package fresh spinach |