Using a Mandoline slicer (or a very sharp knife), slice the 2 large yellow onions sliced into very thin rings on a mandoline into paper-thin rings, about 1/8-inch thick. Separate the rings and place them in a Mixing Bowl of cold water for 10 minutes — this removes excess sharpness and helps them crisp up when fried. Drain well and pat completely dry with Kitchen Towel. Moisture is the enemy of crunch!

Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.

Pour the 4 cups vegetable oil or canola oil, for deep frying into a deep, heavy Pot and heat over medium-high heat to 375°F. Working in small batches, dredge the dry 2 large yellow onions sliced into very thin rings on a mandoline rings first in the seasoned 1 cup all-purpose flour divided — half for dredging, half for batter-0.5 cup cornstarch mixture, then dip into the 1 cup buttermilk cold batter, letting any excess drip off. Carefully lower into the hot oil and fry for 2–3 minutes, turning once, until deeply golden and ultra-crispy. Do not overcrowd the pot — this drops the oil temperature and makes soggy chips.

Use tongs to transfer the fried 2 large yellow onions sliced into very thin rings on a mandoline ring chips to a Wire Rack set over a Baking Sheet (not Kitchen Towel — the rack keeps air circulating so they stay crispy). Immediately season with a pinch of 1 ½ tsp kosher salt plus more for finishing while hot. Repeat with remaining rings, returning the oil to 375°F between batches.