Caramelize the onions by heating the butter or olive oil in a large skillet over medium heat. Add the onions and cook, stirring often, for 15 minutes, or until golden brown and tender. Side Note: Add a little bit of water if the onions run dry while cooking.
Season the onions with sea salt to taste. Remove from the skillet and set aside.
Cook the tostada shell by heating the olive oil in the same large skillet over medium-high heat. Add the corn tortilla and cook for 3 minutes per side, or until golden brown and crisp. Remove from the skillet and place on a paper towel to drain. Season with sea salt to taste.
To create the pork tostada, spread the warm beans over the crisp tortilla, then add the warmed shredded pork followed by the mixed greens, cheese, tomatoes, and green onions.
Serve immediately with salsa verde, guacamole, and sour cream. Enjoy.