Heat olive oil over medium heat in a large frying pan. Saute onion and garlic until soft. Add spinach and stir to coat with oil. Cook until spinach is wilted. Remove from heat and mix in feta. Season to taste with salt & pepper. Preheat oven to 350F. Lay out a sheet of phyllo, brush with melted butter and sprinkle with panko. Place another sheet of phyllo atop the panko, pressing down the phyllo to be sure it sticks. Repeat until you have used three sheets of phyllo. Brush the top sheet with butter, then divide the stack in quarters, crossways. Place a small amount of filling on each of the four strips, and start folding it up in triangles. To do this, take the bottom right-hand corner and fold it so that the bottom straight edge is lined up with the left straight edge. Next, take the bottom left corner and fold it so that the left straight edge is lined up with the top straight edge. Repeat for each square. Place triangles on a parchment-lined sheet pan and bake for about 20 minutes, until golden brown and hot all the way through. Makes 4-6 servings.