Whisk together 3 parts oil to 1 part acid (olive oil and lemon juice or vinegar) with a teaspoon of Dijon mustard for emulsification. Add a touch of honey or maple syrup to balance acidity and salt to taste.
Basic dressing:
Toss with baby spinach just before serving. Add-ins like thinly sliced red onion, toasted nuts, crumbled cheese, or fruit can round out the salad.
Make ahead and store in the fridge up to a week; shake before using. This quick vinaigrette keeps spinach crisp and flavorful without overpowering the delicate leaves.