Broccoli and white cheddar soup

2 tablespoons butter;

1⁄2 cup chopped carrot;

1⁄2 cup chopped celery;

3⁄4 cup chopped red onion;

1 cup diced redskinned potato;

2 large broccoli stalks, florets cut into bitesized pieces, stems peeled and finely chopped (about 4 cups);

3 tablespoons instant or all-purpose flour;

2 cups chicken stock;

1 tablespoon fresh or 1 teaspoon dried thyme;

11⁄2 teaspoons dry mustard;

11⁄2 teaspoons kosher salt;

2 cups whole milk;

11⁄2 cups grated sharp or extra-sharp white cheddar cheese;

2 teaspoons cornstarch.

Gradually stir in the stock and cook, stirring, until the liquid begins to thicken, about 1 minute. Stir in the thyme, mustard, salt, and milk. Simmer until the vegetables are tender, about 20 minutes.
Meanwhile, toss the cheese and cornstarch together in a small bowl. Remove the pan from the heat, add the cheese mixture, and stir until the cheese melts. Check the seasoning and add more salt, if needed. Serve hot.
Make-ahead note: You can make the soup up to 3 days ahead. Cool, cover, and refrigerate. The soup thickens as it cools. If it is too thick when gently reheated, add a little more stock or milk. Do not let it boil.