Q. 2. See question above. (If I bought a red onion that is very big, and I cut only a piece of it…). A fish salad has sliced eggs on top/side, is it alright to eat the sliced eggs the next day?
3. I Also read that a red onion has a different status to a white onion, so can I leave half in the fridge overnight?
All the best,

A. Yaskil Abdi (O. H. 44) maintains that if someone prepared an egg early in the morning, before dawn, to eat later in the day it should be considered as being left overnight, since the food passed from night into day. However, it would be permitted when left prepared with other ingredients, such as oil or mayonnaise.
By the same token Horav Shlomo Miller’s Shlit’a opinion is the eggs in the fish salad are part of other ingredients and therefore permitted.
In regards to an onion being a dovor chorif or a sharp pungent food some Poskim maintain that red onions are milder (See Pischei Teshuva and Kaf Hachaim Y.D. 96: 3 – Mikra’ei Kodesh, Pesach, I:80:3, suggests that some varieties of onions are not chorif at all.
However, in regards to onions left overnight Horav Shlomo Miller’s opinion is that there is no difference between onions.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a