|Unsalted butter||50 Milliliter (1/4 cup)|
|Onion||1 , finely chopped|
|Mushrooms||1 Pound, finely chopped (500 gram)|
|Shredded swiss cheese||250 Milliliter (1 cup)|
|Freshly grated parmesan cheese||125 Milliliter (1/2 cup)|
|Fresh bread crumbs||125 Milliliter (1/2 cup)|
|Chopped fresh parsley||2 Tablespoon|
|Dried tarragon||1 Teaspoon|
|Freshly ground pepper||1⁄4 Teaspoon|
|Mayonnaise||75 Milliliter (1/3 cup)|
|Fresh spinach leaves||8 Large|
In skillet, melt butter; cook onion for about 5 minutes or until softened but not browned.
Add mushrooms and cook until mushrooms are softened and liquid has evaporated; let cool.
In large bowl, blend together eggs, Swiss cheese, Parmesan, bread crumbs, parsley, tarragon, salt and pepper; stir in cooled vegetables.
Pour into buttered 8 inch square (2 L) baking dish and bake in 375Â°F (190Â°C) oven for 30 to 35 minutes or until top is puffed and golden.
Cut frittata into 6 pieces and split each piece horizontally.
To assemble sandwiches, spread mayonnaise over bread slices.
Place 1 or 2 frittata slices on half the bread slices, top with spinach and cover with remaining bread slices.
Wrap securely with plastic wrap or waxed paper.