For The Potato Gruyere Crust: Preheat oven to 350º F. Place potatoes in a large pot and cover with water by 3 inches. Bring to a boil over high heat. As soon as the water boils, set your timer for 30 minutes. Transfer potatoes to a plate to cool for at least 30 minutes.
When the potatoes are cool enough to handle, peel them using the edge of a knife – the peel will easily slide off and in some cases you can even pull strips of it off with your fingers.
Use the largest holes in a box grater to grate potatoes into a large bowl. Add yogurt, mustard, 1 teaspoon of salt, ½ teaspoon Aleppo pepper and ¼ of the grated cheese. Mix well with your hands to create a cohesive “dough.”
Generously butter the cups of a 12-cup muffin tin. Divide the potato mixture into 12 equal portions. An easy way is to divide it first into quarters. Then pat or roll each quarter into a thick log and cut into thirds. Roll and squeeze each portion into a ball. Pat it flat, into a 3-inch disc. Lay it over a muffin cup and gently push it down into the cup, shaping and smoothing it until you have a little cup-shaped crust of even thickness. Repeat with the remaining potato mixture. Bake in the middle of the oven for 25 minutes.
For The Cauliflower Filling: Melt 2 tablespoons butter and 1 tablespoons of oil in a large skillet or heavy pot with a lid, over medium heat. Add the onion and garlic and cook, uncovered, for 5 minutes or until translucent, lowering the heat if they start to brown. Add chopped cauliflower, ½ teaspoon of salt and 1 teaspoon Aleppo pepper. Cook, stirring occasionally, for 5 minutes. Pour 2 tablespoons of water into the skillet, cover and steam cauliflower for 5 minutes. Transfer cauliflower mixture to a large bowl and cool for at least 15 minutes.
In a small bowl whisk eggs and milk. Stir in remaining cheese. Add egg/cheese mixture to the cooled cauliflower and stir to combine.
Spoon the filling into the little crusts. It’s ok to heap it up a bit. It will shrink when it cooks. Spoon any remaining egg evenly over the tarts. Cook tarts in the middle of the oven for 30 minutes. Remove muffin tin from the oven. Turn the oven up to broil. Mix bacon and Parmigiana cheese in a small bowl or cup. Sprinkle mixture evenly over the tops of the pies. Return tin to the oven for just a minute or two to heat the topping and finish browning the crust, checking every minute to prevent burning.
Let tarts cool in the tin for at least 20 minutes for easier un-molding. To un-mold, run a knife around the edge of each tart. Use the side of the knife to nudge the pie up and then gently pull it out of the tin. Serve tarts warm or at room temperature.
*Make Ahead Tip: Potatoes can be boiled, cooled, covered with plastic wrap, and stored in the fridge overnight. Then just peel, grate and proceed with the recipe.