Trim the ends of the asparagus. Set aside. Peel onion and cut into ½ inch circles, keep the layers together.
Heat a grill to high heat. Coat asparagus and onions olive oil and season with salt and pepper.
Place asparagus and onions on the grill. Turn occasionally for 3-5 minutes. Asparagus and onion will be tender and slightly charred.
Remove asparagus and onion from the grill. Allow to cool a few minutes and chop into large pieces.
On a bed of salad green, top with asparagus, onion, chopped roasted red pepper, halved tomatoes, and crumbled feta.
Squeeze the juice from 1 lemon over the salad (about 1 ½ tablespoon of fresh juice). Toss salad to mix ingredients, and serve.