Preheat the oven to 400 degrees . In a small saucepan, cook the butter and flour over medium-low heat, whisking, for 1 minute. Whisk in the milk, then season with salt and pepper. Remove from the heat and whisk in 1 tablespoon mustard.
Slather a quarter of the white sauce over each baguette quarter, then top with the ham, gruyere and thyme. Bake until the cheese is bubbly, about 12 minutes.
Meanwhile, in a small bowl, combine the shallot and vinegar. Stir in the remaining 3 tablespoons mustard, then whisk in the EVOO. In a large bowl, combine the frisee, cucumber and radish; toss with the dressing. Serve the salad with the pizza.