the Ultimate Steak Sandwich

3 heaping tablespoons all-purpose flour

2 cups whole milk

2 cups grated Gruyere

1 tablespoon prepared horseradish Fennel Slaw:

1 large fennel bulb, fronds removed and halved

1/2 lemon , juiced

1/2 cup mayonnaise

3 heaping tablespoons sour cream

1 bag coleslaw mix

2 tablespoons chopped fresh tarragon leaves

2 tablespoons chopped fresh flat-leaf parsley

1 tablespoon extra-virgin olive oil

Kosher salt and freshly ground black pepper