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. Saute 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 1/4th of the sliced potatoes in a shingle-like pattern, being sure the edges overlap. Then add 1/3 of the onion mixture and spread it out evenly over the potato slices. Top this with another 1/5 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. Finish it by broiling on high for 5-7 minutes until golden brown and bubbly.