Preheat the oven to 350 degrees.
In a medium fry pan, melt the butter over medium-high heat, add the onions and chiles and saute until just softened, about 5 minutes.
While the onions and chiles cook, whisk together the eggs and mayo in a small bowl until smooth, then season with the salt, pepper, and garlic powder.
Cut the puff pastry into 12 rectangles, then score about 1/2 inch from each edge, as well as throughout the center of each rectangle, being careful not to cut all the way through the puff pastry. Place on a lightly greased cookie sheet, then spoon about 2-3 Tbsp of the egg mixture into the middle of each rectangle. Add a spoonful of the chiles and onions, then sprinkle with the diced tomatoes. Top with a hearty pinch of white cheddar.
Bake for 25-30 minutes or until the edges are golden brown and the eggs have set. Enjoy immediately.