Add the avocado, oregano, and lemon juice to a mortar (if you don’t have a mortar, add the ingredients to a medium bowl) and season with salt and pepper. Using a pestle, or a fork, mash the ingredients into your desired consistency. Set aside.
Season the tomato slices with salt and freshly ground black pepper. Heat 1/2 tablespoon olive oil in a medium cast-iron skillet over medium-high heat. Add the tomato slices and cook, turning once, for 5 minutes. Remove from the heat, transfer the tomatoes to a plate, and set aside.
Toast the bread. Put the bread on an individual plate. Drizzle a little bit of olive oil over the bread and spoon the avocado evenly on top. Add some of the onion and arrange the tomatoes on top of the avocado. Sprinkle with the remaining onions and garnish with oregano. Season with a pinch of salt and a generous amount of black pepper. Drizzle with additional olive oil, and serve with spring peas on the side, if desired. Serve immediately.