Place asparagus stalks in a steamer over 1 inch of boiling water; cover and wait until they're bright green and tender, 3-5 minutes.
When cool, cut into 1/2-inch pieces. Purée asparagus in a food processor or blender. Add avocado, Serrano, lime juice, and garlic, and pulse until guacamole is desired texture (I like mine creamy, others prefer chunky). Stir in red onion, cilantro and tomato. Season with salt and pepper.