This is a simple salad that can pair with a dish such as Wine-Braised Chicken.
Preheat oven to 350 degrees. Make dressing: In a large bowl, whisk together mustard, vinegar, and 3 tablespoons oil. Season with salt and pepper. Mix in onion; let stand for 15 minutes to soften.
Meanwhile, make croutons: In a medium bowl, combine remaining tablespoon oil, bread, and thyme. Season with salt and pepper; toss. Arrange in a single layer on a baking sheet; bake until golden, 10 to 15 minutes. (If making ahead, store in an airtight container at room temperature up to 1 day.)
Add lettuce and celery to bowl with dressing; toss. Divide among serving plates, and serve topped with croutons.