Heat the olive oil in a large sauté pan over medium heat.
Add onion and cook until soft.
Add mushrooms and cook until they have released their liquid. Cook off the liquid and allow mushrooms to brown.
Add the garlic and sauté 30 seconds more.
Add drained spinach and stir until hot.
Add herbs, salt and pepper to taste, cool until warm.
Add beaten egg and feta, stir to combine.
Assemble
Keeping filo covered, separate 1 sheet and brush with melted butter. Repeat with 3 more sheets.
Cut in half lengthwise into two strips.
Place 1 #15 scoop of filling onto an end of each strip.
Fold into triangles and move to baking sheet. Brush each with more butter.
Bake:
Bake until golden, check after 10 minutes. Rotate pans as necessary, should take 20 to 30 minutes total.