Before Lent, I bought Nature's Path Organic Apple Cinnamon Oatmeal. It's this stuff. The ingredients are simple: Rolled oats, cane sugar, dried apple, cinnamon, and sea salt.
I know they had all these ingredients in the middle ages/Renaissance, but did they have oatmeal as we know it. Yes. Yes, the did.
1520's Spain Oatmeal Gruel
1390's England Grewel of Almounds
1420's France Oatmeal
There are actually quite a few different recipes but almost all have the basic oats in almond milk (or regular milk) with salt and sometimes sugar. They'd add all sorts of things really beyond that so apple and cinnamon? Not exactly out there for the middle ages.
Luckily for me, this means I get to have oatmeal for breakfast on non-fasting days! And without soaking the oats for days or any of the other work that sounds way too hard for just making a bowl of oatmeal.