Melt the butter in a heavy saucepan, and sauté the prawns, spring onion tops, mushrooms, garlic, spinach, salt and lemon juice for 7–10 minutes until the vegetables go limp.
Spread out the tortillas, and divide the seafood mixture equally between them, spooning it over the middle of each one. Roll up each tortilla, and tuck in the ends to form solid packages. Secure with toothpicks.
Heat the oil in a deep heavy saucepan. Fry each chimichanga for about 3 minutes until golden, turning it over as it cooks to brown on all sides.
Remove from the oil and drain on paper towels. Serve hot with cheese, guacamole, sour cream and diced tomatoes.