Preheat the oven to 375°F (190°C).
Heat 2 tablespoons butter in a skillet over medium heat.
Add the chopped onion and the small pieces of chicken; cook, stirring occasionally, until the chicken is no longer pink and the onion is softened, about 6–8 minutes. Season with salt and pepper to taste.
Stir in the chopped red bell pepper and corn kernels; cook 2–3 minutes more until the pepper softens slightly. Transfer the mixture to a bowl and let it cool briefly.
Once slightly cooled, stir in the crumbled feta and chopped green onions until evenly combined.
Unroll one sheet of phyllo on a work surface and cover the remaining sheets with a damp towel to prevent drying. Brush the sheet lightly with melted butter and place a second phyllo sheet on top; brush that sheet with butter as well.
Using a pizza cutter or sharp knife, cut the layered phyllo lengthwise into four long strips (about 2 inches wide).
Place a heaping tablespoon of the chicken filling near one corner of a phyllo strip. Fold the corner over the filling at a 45° angle to form a triangle, then continue folding the strip in triangular folds (like folding a flag) until you reach the end. Repeat with remaining strips and filling, re-buttering and layering phyllo sheets as needed.
Arrange the filled triangles on a baking sheet, brush the tops with the remaining melted butter, and bake for 20–30 minutes until golden brown and crisp.
Remove from the oven and let cool slightly before serving.