Gently shape the ground chicken into a burger patty, being careful not to overwork the meat. Season both sides generously with salt and freshly ground black pepper.
Heat the avocado oil or olive oil in a skillet over medium heat. Add the chicken patty and cook with the lid on for about 12 minutes, flipping it a couple of times, until both sides are nicely browned and the chicken is fully cooked through. The centre should reach 74°C/165°F.
Lightly butter the cut sides of the burger bun. Place them cut-side down in a separate pan over medium heat and toast until golden and crisp to your liking.
In a small bowl, stir together the mayonnaise, minced garlic, lemon juice, and paprika. Add a little Sriracha if you’d like some heat.
Spread a generous layer of homemade guacamole over the bottom bun. Add the cooked chicken patty, followed by the sliced feta, plenty of thinly sliced red onion, and a handful of crunchy Boston lettuce. Spread the garlic paprika aioli over the top bun, then place it on top.
Serve immediately while the chicken is warm and the bun is freshly toasted.