1. 1. In a small bowl, combine the thinly sliced red onion, lime juice, and sugar. Stir well and set aside for at least 20 minutes to allow the flavors to meld and the onions to pickle.

2. 2. Toast the sourdough bread slices until lightly golden and crispy.

3. 3. Spread an even layer of basil pesto on each toast.

4. 4. Divide the baby spinach evenly among the toasts, placing it on top of the pesto.

5. 5. Arrange the sliced avocado on top of the spinach.

6. 6. Place the aged cheddar cheese slices over the avocado.

7. 7. Heat the olive oil in a large frying pan over medium heat. Carefully place the assembled toasts in the pan and fry for 3-4 minutes on each side, or until the bread is golden brown and the cheese begins to melt. Be cautious when handling the hot pan and flipping the toasts.

8. 8. Using a spatula, transfer the fried toasts to a cutting board. With a sharp knife, carefully cut each toast diagonally in half.

9. 9. Using tongs, place the pickled red onions on top of each toast half. Serve the avocado toast warm and enjoy your delightful brunch creation. Remember to handle the hot toasts with care to avoid burns.