Whisk 1/4 cup milk, flour, mustard, salt and pepper in a small bowl; place near the stove. Add the remaining 1 1/2 cups milk to the saucepan. Heat over medium-high heat until just simmering. Add the flour mixture and reduce heat to medium-low. Cook, whisking constantly, until thickened, 1 to 2 minutes. Remove from heat and whisk in Cheddar, cream cheese and vinegar until the cream cheese is melted. Stir the pasta and kohlrabi into the sauce. Serve topped with salami.