Place the avocado and lemon juice in a bowl. Use a fork to coarsely mash.
Use a large serrated knife to cut the bread into 2 pieces, then cut each piece horizontally to make 4 thin pieces. Place, cut-side up, on a baking tray.
Place the chicken in a large shallow dish. Add the paprika, oregano, garlic and oil. Rub spice mix over the chicken to coat.
Heat a large non-stick frying pan over medium-high heat. Add the chicken and cook for 3-4 mins each side or until golden brown and cooked through. Transfer to a large plate. Loosely cover with foil to keep warm.
Add the capsicum to the pan and cook, stirring occasionally, for 3 mins or until lightly browned and just tender. Transfer to the plate with the chicken. Add pineapple to the pan. Cook for 30 secs each side or until lightly caramelised. Chop.
Preheat the grill. Thickly slice the chicken. Cook the bread under the grill until toasted. Spread the toast evenly with avocado mixture. Top with capsicum, chicken, pineapple and cheese. Cook under the grill for 1-2 mins or until the cheese melts and is golden.
Divide the melts among serving plates. Serve with the salad.