Makes 2 servings.
accompaniment: soy sauce for dipping, if desired
In a bowl whisk beer into flour until smooth and stir in salt. Make several shallow cuts across inside curve of each shrimp and press shrimp gently (to help prevent curling during cooking). Peel sweet potato and cut crosswise into 1/4-inch-thick slices.
In a 3-quart saucepan heat 2 inches oil to 375°F. 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 tempura with soy sauce.