Trim and cook nopales:
Put a cactus paddle flat side down on a work surface. Scrape a knife almost horizontally from narrow base of paddle toward wider end to cut off thorns. Dethorn paddle on other side, then trim all around edge. Repeat with remaining paddles, then cut into 1/2-inch cubes.
Cook onion in 1 tablespoon oil in a large heavy skillet over moderate heat, stirring, until softened. Add cactus and 1 teaspoon salt and cook, stirring, until well coated with oil, about 1 minute.
Reduce heat to low and cook, covered, stirring occasionally, until cactus is crisp-tender and has oozed liquid, about 5 minutes. Increase heat to moderate and cook, uncovered, stirring occasionally, until liquid is evaporated, 7 to 9 minutes.
Toast pumpkin seeds:
Cook seeds in a dry 12-inch heavy skillet over moderate heat, stirring constantly, until fragrantly toasted, 8 to 12 minutes. (Some seeds will be visibly puffed and possibly open but not significantly colored.) Spread seeds on a tray to cool.
Make chile purée:
Cover chiles with cold water in a saucepan and simmer 5 minutes, then let soak off heat 5 minutes. Drain in a colander, then purée chiles in a blender with garlic and 1 cup water until very smooth, at least 1 minute. Force purée through a sieve into a bowl, pressing hard on solids, and discard solids.
Heat remaining 3 tablespoons oil over moderate heat in a deep 12-inch heavy sauté pan until hot but not smoking, then fry chile purée, stirring constantly, until reduced slightly, about 2 minutes.
Finely grind pumpkin seeds with cumin in small batches in an electric coffee/spice grinder. (It will look like a powder but will feel like sand.) Transfer to a bowl and stir in remaining 3 cups water, then force through a fine sieve into chile purée, pressing hard on solids, and discard solids. Simmer sauce, stirring occasionally, 20 minutes (it will thicken).
Add cactus, zucchini, epazote, and remaining teaspoon salt and simmer, stirring occasionally, 5 minutes.
For a heartier meal, thin sauce with 1 1/4 cups water and return to a simmer, then add eggs (break each egg into a cup first, then slide in), spacing them evenly. Simmer, covered, until cooked to desired doneness, about 10 minutes for runny (not salmonella-safe) yolks.
Available in Latino markets and some supermarkets.
Available in Mexican markets.
Available by mail order through Chile Today - Hot Tamale, (800) 468-7377.