Cook sweetpotatoes, 1/2 white onion and 2 whole jalapeños in a crock pot on low for 6-8 hours. Once cooked, peel sweetpotatoes of their skin and add in a large pan, reserve onion and jalapeño. Mix in 1 cup of vegetable broth and mash until smooth. Salt to taste. In a separate pan, drizzle with olive oil and add the reserved onion and jalapeño, finely chopped (remove seeds). Add drained black beans to pan and cook until warm and liquid is reduced. Take a tostada from the package and smooth a layer of sweetpotato on the top of the tostada. Then top with a small spoonful of black bean mixture, a spoonful of pico de gallo and finish with crumbled cotija cheese. Serve warm. Continue building tostadas for each guest using the same steps outlined above.