I have been known to make this dish for dinner, however I particularly love this version of gratin for brunch or even breakfast. It's great on days when you need something with a bit of substance for your early meal and the perfect weekend morning treat! What's also great about this dish is that you can prepare it ahead of time and store it covered with cling film in the refrigerator overnight. If possible, remove from the fridge and uncover about an hour or so before baking to allow it to come up to room temperature.
I haven't given you an exact measurement for the Polish sausage because to be honest it doesn't matter if you use more or less. I have found that generally the links vary in size and it seems silly to waste what's left over or not make this dish because you don't have quite enough. Any amount will do, but what you're looking for ideally is enough to layer into the pan twice when thinly sliced. It's also worth saying that you're looking for a Kielbasa here, but there are many different types, so just use what's either available to you or your favorite. There's no wrong answer here that I know of...
2x banana shallots, finely chopped
25g unsalted butter, plus more for greasing & finishing
1 tablespoon regular olive oil (not extra-virgin)
250g potatoes, unpeeled & thinly sliced (preferably with a food processor)
Polish sausage, thinly sliced (approximately 1 footlong link)
3 eggs, lightly beaten
275ml single cream
50g grated Gruyère cheese
Sea salt flakes & freshly ground black pepper
1. Preheat the oven to 200°C. Grease a casserole dish, baking pan or pie plate with a bit of of butter and set aside. You want something like an 8 inch square pan or similar such as a 7x9 rectangular casserole. Additionally, put on a large saucepan of water to boil.
2. In a non-stick frying pan, melt the butter and olive oil together and cook the finely chopped shallots over medium-low heat for 5 minutes or so until tender being careful not to burn them. When they are done, set them aside to cool.
3. Cook the potato slices in the boiling water for about 5 or 6 minutes until they're kind of soft, but not quite done. Then drain the potatoes when they're done.
4. Mix the beaten eggs, cream, a few grinds of pepper, a pinch of sea salt flakes and the shallots in their cooking butter together gently with a fork or wire whisk.
5. To assemble the gratin, put one layer of potatoes on the base of the buttered dish followed by a layer of polish sausage. Then ladle some of the creamy egg & shallot mixture over the top and repeat until all of the potatoes and sausage slices have been used. You should get at least 2 layers. If you have additional sausage left over, you can always use it on the top of the last layer in the following step.
6. Once you've finished layering and you've poured the remaining cream and egg mixture on top (all of this should be used) finish by sprinkling the cheese over top. You'll want to use all of it. Finally, season with salt & pepper and dot the top with tiny little chunks of butter about the size of a pencil eraser.
7. Bake in the oven for 30 to 40 minutes until the top is bronzed and the cheese is fully melted.