Look for potatoes that are oblong and don't taper too thinly towards the ends (uniform diameter). If you have a mandolin and aren't afraid to use it, it makes cutting the potatoes much easier. Otherwise, use a sharp knife and cut the potatoes as thinly as possible, while maintaining a full slice. Be sure to test for doneness by fishing out a potato slice from the centre of the pan and tasting. Finally, be sure to salt the sauce well as it's the main way that salt is introduced in to the dish.
1/2cuponionthinly sliced, cut in to 1-inch long pieces and separated
6-7largeYukon Gold or other yellow-fleshed potato
1 1/2cupswhole milk3 or 3.5%
Splash of Worcestershire sauce
1/2tspfresh thyme leavesoptional
1tspfine sea saltmore as needed
Freshly ground pepper
100gAged White Cheddar3.5oz, shredded
Preheat oven to 375° F. and grease a rectangular, shallow baking pan approximately 8x10-inches (or 9x9 inches would work, as well)
In a skillet, over medium heat, cook bacon until lightly browned. Remove to a paper-towel to drain and set aside.
In a medium saucepan over medium heat, melt butter. Add flour and cook, stirring, for about 1 minute (mixture will be crumbly). Add milk and chicken broth and whisk, until well combined. Cook until mixture is smooth and thickened. Remove from heat. Add mustard, Worcestershire, thyme leaves (if using), salt and pepper. Taste sauce. Sauce should be well-salted, so add more, if necessary. Put lid on saucepan and set aside.
Cut ends off potatoes. Slice into thin rounds (as thin as you can while still getting a complete slice), using a sharp knife or a mandolin. Stand the potatoes up side-by-side in your prepared pan. Continue until you have about 3 rows that fill up 3/4 of the width of the pan (by the time we put onion and bacon in between, they will spread out to fill the entire width of the pan).
Take your sliced onion and start inserting it in between every 3 or 4 potato slices or so. It doesn't need to be too precise. Just fit some in here and there. Repeat with the bacon bits, trying to insert in different spaces. Spread your potato slices out evenly across the entire width of the pan, ensuring they are still standing up-ish. Sprinkle with some salt and freshly ground pepper.
Pour our sauce over top. Using the back of a spoon, push the sauce from the top of the potatoes down in between the potatoes and between the rows.
Cover with foil. Bake on a baking sheet (in case of bubble overs) for 45 minutes covered. Remove foil. Sprinkle grated cheese evenly over top. Return to oven uncovered (still on baking sheet) and cook for an additional 20-30 minutes. Test potatoes for done-ness by sampling a potato slice from the centre of the dish and making sure it's cooked. If not, cook a bit longer.