To make the Brodetto, in a medium saucepan warm up oil over medium heat. Add diced onions and saute until they are softened.
Add the lemon juice, zest, and broth. Bring to simmer over low heat.
To make the Pea Puree, combine the peas, mint, garlic, salt, and pepper in a blender or a food processor; puree the ingredients. With the machine running, add the olive oil to the peas. Transfer the pea puree to a small bowl and set aside.
To make the salmon, warm the olive oil in a large skillet over high heat. Season the salmon pieces with salt and pepper. Sear the salmon until a golden crust forms, about 4 to 5 minutes on the first side. Flip the fish and continue cooking for about 2 minutes or more, depending on the thickness of the fish. You would be able to see on the sides of the salmon when it starts cooking through.
To put the dish together, add 1/3 of a cup of Brodetto. Place a large spoonful of puree into the center of each plate. Place the salmon piece on top of the sauce; repeat for every plate.