Slice 2–3 large yellow onions thinly (about 1/8 in / 3 mm).
Heat 2–3 tbsp butter and/or 2 tbsp olive oil in a large skillet over medium-low heat.
Add onions and stir to coat.
Season with 1/2 tsp salt and optional pinch of sugar (optional for faster browning).
Cook, stirring every 5–10 minutes, adjusting heat to keep onions gently sizzling (about 30–60 minutes).
Add 1–2 tbsp water or broth if the pan gets too dry or onions start sticking.
Continue cooking until deep golden brown and soft (avoid burning).
Stir in optional flavorings near the end (1–2 tsp balsamic vinegar, thyme, black pepper, or a splash of soy sauce).
Taste and adjust salt.
Use immediately or cool and store refrigerated up to 5 days.