Slice each avocado in half lengthwise, pop out seed and slip off the skin. Smash the avocado in a bowl and add onion, chiles, cilantro, lime juice, tomatoes, and garlic and mix well. Season with salt and pepper, cover, and let sit in refrigerator for a couple of hours to allow flavors to combine.
Lay out tortillas and sprinkle about 1/8 cup cheese on each. Begin heating vegetable oil to 350 degrees F in deep-fryer.
In a large skillet heat olive oil over medium-high heat and saute onion until translucent. To this pan, add ground beef, tomato paste, chili powder, cumin, celery salt, and salt and pepper, and cook until browned. Divide meat mixture into equal amounts in the pan. Then using a slotted spoon to leave excess liquid behind, transfer meat mixture to the center of each tortilla and fold the tortilla into a closed packet and secure using a wooden skewer or round toothpicks. Deep fry chimichangas until golden brown. (Alternatively, you can skip the deep frying step altogether by arranging the chimichanga packets on a baking sheet and bake until cheese melts and tortillas begin to firm up, about 5 to 10 minutes in a preheated 350 degrees F oven.) Remove skewers/toothpicks before serving.