Joy to the Food

0 Preheat the oven to 350 degrees and grab a 9x9 pan or 9x11 casserole dish. Thinly slice your potatoes into ¼” slices, skin on, and thinly slice the onion. This can be done with a mandolin on the #2 setting or with a knife.
1 Heat a medium saucepan or skillet over medium heat and add the margarine and rosemary. Melt this down while stirring constantly until the margarine starts to bubble and turn a slight gold brown. You will start to notice little brown flecks in it, at this point remove it from heat.
2 While that browns, separate the yolks and egg whites. Whisk the egg yolks with the lemon juice and mustard in a medium bowl and set aside. Save the egg whites for another recipe.
3 Once the margarine is done, remove the rosemary and slowly pour it into the egg yolk mixture while constantly whisking so as to not scramble the eggs. Once combined - set aside. This is the emulsifying process!
4 Add the onions, brown sugar, salt, and pepper to the same pan that was used to brown the margarine. Sauté this together until fragrant and soft - about 2-3 minutes. Remove from heat.
5 While the onions are cooking, grab a large saucepan and add the vegetable stock. Bring to a low boil and add the egg mixture while mixing constantly so as to not scramble the eggs. Once totally combined, whisk in the flour and continue to cook at a low bowl until thickened and then remove from heat.
6 Pour a small amount of the sauce into the bottom of the baking dish and spread it out evenly. Top with ¼th of the sliced potatoes in a shingle-like pattern, being sure the edges overlap. Then add ⅓ of the onion mixture and spread it out evenly over the potato slices. Top this with another ⅕ of the sauce and spread it out evenly.
7 Repeat this layering so there are three layers of potatoes, onions, and sauce. Then, add a fourth layer of potatoes and top with the remaining sauce.
8 Bake covered with foil for 35 minutes, then remove the foil and bake for another 35 minutes. After this second 35 minutes, add the cheese over the top evenly. Finish it by broiling on high for 5-7 minutes until golden brown and bubbly.