Red Endive and Watercress Salad

1 teaspoon Dijon mustard

1/2 teaspoon finely chopped fresh thyme

1/4 teaspoon minced garlic

1/4 teaspoon salt

1/8 teaspoon black pepper

1/4 cup extra-virgin olive oil

1 lb Belgian endive s (4 large), cut crosswise into 1-inch pieces

6 oz red endive or radicchio di Treviso, cut crosswise into 1-inch pieces, or 1 medium round head radicchio, torn into bite-size pieces

2 cups loosely packed watercress sprigs (2 ounces)