Use the proportions below as a guide. You can omit the sausage to make it vegetarian — I almost always make it without sausage these days. If you want to scale it up or down, a good rule of thumb is 1/2 teaspoon kosher salt (Diamond Crystal brand) for every 4 eggs… use even less if using Morton’s Kosher salt. 

Update: I’ve increased the oven temperature from 250ºF to 325ºF — when I made this most recently at the lower temperature it took way too long to cook, though my oven has been acting up. If you’ve had success at the lower temperature, go for it. Otherwise, try 325ºF, which will have this casserole done in just about 30 minutes. 

Also: I now add a cup of half and half. I think including some sort of milk product — milk, cream or half and half — makes the eggs more forgiving. When you overcook this casserole when it is eggs alone, it can be a bit rubbery tasting. When dairy is added, I find it remains more custardy. When I think about my favorite egg custard dish, Tartine’s Quiche, which calls for 2 cups of dairy (milk and crème fraîche) for 5 eggs, it makes me want to try this with yet another cup of half and half. I will be sure to report back when I do. 

Finally: When I make it vegetarian, I simply sauté the onions for about 10 minutes; then fold in the spinach just until it wilts. Then I proceed with the recipe as usual.