Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with parchment paper.
Add sausage to a large skillet over medium-high heat. Cook and stir with a spatula, breaking apart the sausage until cooked, 5 to 7 minutes. Reduce heat to medium. Add spinach, green onions, and red pepper flakes. Continue cooking until the spinach is thawed, 3 to 5 minutes.
Remove from heat and stir in ricotta cheese and feta cheese. Season with salt and pepper.
Unwrap the phyllo dough and cover with a towel to keep it from drying out.
Working with one sheet at a time, brush 1/2 of the sheet with melted butter. Fold it in half lengthwise. Place a 1/4 to 1/2 cup of filling in the top-left corner. Fold the bottom-left corner over into a triangle shape. Continue folding edge over edge until you reach the end of the dough and a triangle shape is formed. Place the triangle on the baking sheet. Repeat with the remaining ingredients.
Brush all the triangles with melted butter.
Bake in the preheated oven until golden brown, about 25 minutes.
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