In a Dutch oven or large, deep skillet, heat 2 inches of oil over medium to medium-high heat.
In a bowl, whisk together beer, flour, dried spices and some salt.
Douse rings of red onion with lime juice and season with salt and pepper.
To a food processor or blender, add chile pepper and grate garlic directly into the bowl or blender. Add avocado flesh and squeeze in lemon juice - squeeze it cut-side up to keep seeds from falling into processor. Add sour cream, a good pinch of salt and cilantro, and process into a smooth green sauce. Scrape into small serving dish and reserve.
Coat fish pieces in the beer batter and fry in the hot oil to deep golden, 6-7 minutes. Drain on rack and season with a pinch of salt.