In a bowl whisk beer into flour until smooth and stir in salt.
Make several shallow cuts across the inside curve of each shrimp and press shrimp gently ( to help prevent curling during the cooking).
Peel sweet potato and cut cross-wise into 1/4-inch thick slices.
In a 3-quart saucepan, heat 2 inches of oil to 375 degrees.
Working in batches of 3 or 4 pieces, dredge shrimp, sweet potato slices, bell pepper rings and asparagus spears in batter to coat completely, letting excess drip off and fry, turning, until golden, about 3 minutes.
Transfer tempura as fried to brown paper with tongs to drain and season with salt.
Serve with dipping sauce (recipe follows).
In a saucepan heat soy sauce, sugar and ginger until sugar dissolves, add rice vinegar and scallions.