I got one word for you: banana. But in all seriousness, I've been known to eat a couple slices of turkey or ham, the kinds cut for sandwiches. Deli meat is quick and something most people have on hand. It's also a better alternative in regards to its sugar content.
I would normally prepare rolled oat soaking in water overnight with ratio 1:2, heat up in microwave in the morning then top on mixed berries and a dash of peanut butter for protein and fat and some nuts or seeds for minerals and vitamins
OWYN protein shakes are a staple for me. I found them when my husband was diagnosed with Crohns because he couldn't tolerate most foods, and OWYN is free of most common allergens. It has the right proportions of macronutrients and not a ton of sugar/additives unlike a lot of protein shakes, and is designed to be a snack or meal replacement. These are a lifesaver for me on bad brain days when cooking is not something I am capable of.
If I don't have time to cook, I have a bowl of plain cheerios with whole milk or greek yogurt. Both are rich in protein and/or fiber