Pasjoli's Ultimate Grilled Cheese Sandwich (Croque Matthieu)

Make the Mornay sauce: Add butter to a saucepan over low heat and let it melt completely. Add the flour a little at a time, whisking constantly until incorporated. Cook the butter-flour mixture, whisking until the roux turns a blond color, about 2 minutes.

Slowly whisk in the milk a little at a time until evenly incorporated; whisk constantly as the mixture will be very thick. Once warm, slowly add the cream while whisking constantly as it thickens, about 4 minutes

Once the mixture is thickened, add 1 cup of the Gruyère cheese. The cheese should melt and be thick (do not thin with additional milk or cream). Cook until fully incorporated and thick, about 7 minutes. Remove from the heat.

Once the sauce is completely cool, fold in the remaining 1/2 cup Gruyère. It should not melt into the sauce. Let cool to a paste. This makes about 3 cups Mornay sauce. Set aside until ready ot use.

Make the caramelized onions: Add the oil to a heavy-bottomed Dutch oven over medium-low heat. Add 1/4 cup of the butter. Add the onions, overloading the pan (you want them snug in there). Give the onions a good stir, flipping the bottom to the top, coating them with the oil and butter. Cover the pot to encourage onions to release their moisture. Lift the lid and stir every 5 minutes for 20 minutes. The onions will quickly reduce to less than half their size. Remove the lid, lower the heat and continue to stir every 10 minutes for 40 minutes, allowing some of the water to evaporate. The color will gradually continue to become darker. Be mindful of the bottom of the pan so that the onions don’t burn.

Once the onions start to darken, add Madeira to the pot to deglaze, using a wooden spoon to scrape bits off the pan. Stir occasionally for another 30 minutes. Once the onions are golden brown, add the remaining butter and continue to constantly stir to avoid any scorching until evenly caramelized. This will take up to several hours. But they’ll also be great after a couple of hours.

When the onions are completely caramelized and deeply golden brown, add the heavy cream. Season with salt and sherry vinegar to taste. This makes 1 1/2 cups caramelized onions.

Alternatively, if you cook the onions in a slow cooker, use a little more liquid. Fill the pot up with onions and set it on low (not more than 25% heat). Stir occasionally. If the liquid evaporates, add a little more.

Make the sandwich: Slice off both ends of the baguette. Cut the baguette in half to create two pieces. Trim the top and bottom crust of each piece to create a flat top. Slice the bread in half lengthwise. Set aside.

Put a large nonstick pan over medium heat. Sprinkle the grated Gruyère cheese directly onto the pan in two rows about the size of the bread.

Place two pieces of bread (trimmed-top-side-down) directly on top of the cheese. Use a rubber spatula to scooch any oozing cheese toward/under the bread as the cheese melts. Reduce the heat to medium low and wait until the cheese is caramelized and crisp, sticks to the bread and is beginning to brown, about 5 minutes.

Transfer the bread slices to a baking sheet and let cool slightly.

Spread ¼ cup caramelized onion onto the melted cheese on one half of the bread. Spread 2 tablespoons Mornay sauce onto the melted cheese of the other half and add 1 ounce of proscuitto on top of the Mornay. Combine the two bread halves together. Repeat with the other two pieces of bread.

Using the same nonstick pan, add 1 tablespoon grapeseed oil over medium heat. When the oil is hot, place the sandwich in the pan then add butter. Allow the butter to melt around the sandwich and baste the sandwich until it is golden and crispy on both sides, about 2 minutes on each side. The inside should be hot. Cut in half and garnish with cut chives.