Heat oil in a large nonstick skillet over medium-high heat.
Add shallot and cook, stirring often, until softened, about 3 minutes.
Add vinegar; cook, stirring often, until mostly evaporated, about 2 minutes.
Antonis Achilleos
Transfer the mushroom mixture to a bowl and set aside.
Coat 1 side of each bread slice evenly with butter and set 4 slices aside.
Place the remaining 4 slices, coated-sides down, on a cutting board.
Top each with 1 slice Swiss cheese, 2 12 tablespoons mushroom mixture and 2 tablespoons Gruyere.
Top with the remaining bread slices, coated-sides up.
Wipe the skillet clean and heat over medium heat.
Add 2 sandwiches; cook until the bottom side is crispy and browned, 2 to 3 minutes.
Carefully turn the sandwiches over and continue cooking until the cheese has completely melted, about 2 minutes more.
(Alternatively, heat a panini press to medium.
Place 2 sandwiches on the cooking grates and shut the press.
Remove from the skillet; keep warm in the oven.
Repeat with the remaining sandwiches.
- Daily Values (DVs) are the recommended amounts of nutrients to consume each day.
(-) Information is not currently available for this nutrient.
