This mini riff on the classic was designed to be picked up, so the creamy, cheesy sauce on top is left out, but you could easily add it. Drizzle it over top, then stick the dish under the broiler until the cheese begins to brown and bubble.
A Mornay sauce is a cheesed-up offshoot of a simple bechamel, which is one of the fundamental mother sauces. For a simple bechamel, make a smooth roux of melted butter and flour, then stir in milk or cream. If you’re making a Mornay sauce, first make a thin bechamel sauce, using 1 Tbsp. each of butter and flour to each cup of milk. Then, to make it into Mornay, melt in 1/2 cup cheese. The Brown Hotel in Louisville, Kentucky, which invented the Hot Brown, uses pecorino Romano, but Swiss and Gruyere work beautifully, too.
You can reheat fully prepared sliders, but if you want to prepare them in advance, it’s best to assemble and put them in the refrigerator. Then, when you’re ready to bake, take them out, make the sauce and then bake as directed.