Spanish tortilla!! No dairy and it is vegetarian, as long as eggs are ok.
https://www.foodnetwork.com/recipes/food-network-kitchen/spanish-tortilla-recipe-1928450
I used this recipe as a basic guide but have many so many times that I’ve simplified the steps considerably. As written it is unnecessarily messy and more complicated than needed. Here is what I do:
I use a 12 in Lodge cast iron skillet. Preheat oven. Heat olive oil in skillet over medium and add 1/4 inch sliced potatoes and onions. Use enough oil that potatoes and onions are mostly covered. Keep heat at medium. You only want the potatoes to get soft and slightly golden. As soon as potato color starts to turn gold, drain out the the excess oil. I just pour it out since that is quickest and easiest. Heavily salt and pepper the potatoes. More than you think. Pour on beaten eggs over potato mixture. Put in oven and cook set and golden brown on top. You can turn broiler on for a minute if it sets before it browns enough.
Done! You can serve warm, cold, or room temp. It is delicious any which way. You can serve directly from the skillet or you can slice it up however you want and put on a platter (slices better once it is cooled). So really, you can make this the day before, or any time day of. Make the sofrito sauce as written. It is a must and delicious. That can also be made anytime ahead.