Place the sliced mushrooms in a frying pan on low heat and dry fry gently until the juices emerge. Add the chilli flakes if desired and lightly fry for a couple more minutes. Set aside.
In a medium-size bowl, roughly mash the avocado with a fork. Add a large squeeze of lemon juice and mash further until just combined. Season with sea salt and freshly ground black pepper.
Pop the bread into the toaster. Spread avocado evenly on the bread and top with rocket leaves, sprouts, pesto and sunflower seeds.
Serve with warm mushrooms.