Heat the processed cheese spread in a heavy saucepan over moderate heat until melted and smooth, stirring frequently to prevent scalding. Reduce heat to low to medium-low. Stir in the scallions, beer and mustard and let come to a gentle simmer. Toss the grated Cheddar with the cornstarch. Slowly whisk in the Cheddar a handful at a time, whisking until each batch is combined before adding more. Season with salt and pepper. Stir in milk, a little at time, to thin if necessary (I prefer mine quite thick). Keep warm in a crockpot or over a sterno, stirring occasionally, in order for dip to remain smooth. Serve immediately with toasted pretzel bread.