While bread toasts, bring a small saucepan of water to a gentle simmer. Add vinegar.
Crack egg into a small bowl. Create a gentle whirlpool in the water and slide the egg in. Poach for 3 minutes for runny yolk, then remove with a slotted spoon and drain on paper towel.
Mash the avocado with a fork and spread onto the toast. Season with salt and pepper.
Place the poached egg on top. Sprinkle with seeds, additional salt and pepper, and red pepper flakes if using. Serve immediately.
Tips
For extra flavor, add a squeeze of lemon juice to the avocado.