Adjust oven rack to upper-middle position and heat broiler on High. Line broiler safe baking dish (7 x 11 pan is best) with paper towels or a lint free kitchen cloth. Slice the woody ends off the asparagus and discard them.
Bring broth to a boil in a large skillet over medium-high heat. Add asparagus stalks to skillet and cook, stirring occasionally, until nearly tender, 2-4 minutes. Transfer asparagus to paper/cloth-lined baking dish. Pour remaining broth into a heat-safe liquid measuring cup. If liquid does not read 1 1/4-cup mark, add water until it does.
Melt butter in now empty skillet over medium heat. Add flour blend and cook, stirring constantly until golden, about 1-3 minutes, depending on your blend. Whisk in reserved broth and bring to boil. Reduce heat to medium-low and simmer until thickened, 3-5 minutes. Off heat, whisk in ½ cup Parmesan and White Cheddar until smooth. Season with salt and pepper.
Cover and let stand for 5 minutes.
Remove paper towels from baking dish. Drizzle sauce over center of asparagus and top with remaining Parmesan. Broil until cheese is golden and asparagus is tender, 4-8 minutes. Serve.
Also delicious with sliced red peppers added in with asparagus.