¾ cup finely chopped celery
¾ cup peeled and finely chopped carrots
¾ cup chopped scallions
3 ears fresh corn, shucked
1 lb. Yukon Gold or other all-purpose boiling potatoes, peeled and cut into ½-inch cubes
4 oz. bacon (about 5 pieces), cut into ½-inch pieces
6 Tbsp. unsalted butter
¾ cup peeled and finely chopped yellow onion
6 Tbsp. all-purpose flour
2 cups milk, warmed
2 ¼ to 2¾ cups chicken stock, warmed
½ lb. extra sharp Cheddar cheese, finely grated
1 tsp. ground mustard
Salt, cayenne pepper and freshly ground black pepper to taste
Additional grated Cheddar cheese and chopped scallions for garnish (optional)
Fill a medium saucepan with 3 cups lightly salted water. Bring it to a boil. Reduce the heat to medium and add the celery, carrots and scallions. Cook until tender, about 5 minutes. Drain and reserve the vegetables.
Fill a large (6- to 7-quart) stockpot with 4 quarts lightly salted water. Bring it to a boil over medium-high heat. Add the corn ears and blanch them 2 to 3 minutes. Remove the corn from the water and set it aside to cool.
While the corn is cooling, bring the water back to a boil, as needed, and add the potatoes. Cook them over medium heat until they are tender, but not mushy, about 7 to 8 minutes. Drain and reserve the potatoes.
Once the corn has cooled enough to handle comfortably, slice the kernels off with a sharp knife; you should have about 1½ cups.
Clean the stockpot and place it over medium heat. Add the bacon. When the bacon has rendered a few tablespoons of fat, increase the heat to medium-high and cook the bacon until crispy, stirring as needed to prevent sticking. Remove the bacon with a slotted spoon and set aside. Pour off all of the fat.
Add the butter and return the pan to medium heat. When the butter has melted, add the onion and cook until softened but not browned, about 7 to 10 minutes.
Remove the pan from the heat and blend in the flour to create a roux. Return the pan to medium heat and cook the roux for 2 to 3 minutes, whisking regularly. Gradually add the milk and stock, whisking constantly to prevent lumps and possible scorching. Stop adding the stock when the soup has reached your ideal consistency.
Bring the mixture to a boil and allow it to boil just a few minutes, until slightly thickened. Immediately reduce the heat to low and add the cheese, bacon and reserved potatoes and vegetables. Stir gently to combine. Continue to cook, without boiling, until the cheese has melted.
Add the mustard and season to taste with salt, cayenne pepper and black pepper. Serve immediately with a sprinkling of grated Cheddar and/or chopped scallions.