Make the broth: Mix the bouillon paste, tamari, and water together. Set aside.
Sauté the onions: In a large skillet over medium-high heat, add a big drizzling of olive oil. Once hot, add the onions and a sprinkling of salt and pepper. Mix well and cook until golden and tender, about 8 minutes, stirring every so often.
Make and cook the garlic-herb paste: While the onions cook, in a small pile on your cutting board, combine the chopped garlic, oregano, and a sprinkling of sea salt. Chop this up until well combined. Add to the onions (once they are golden), along with the butter, and cook for an additional 1 minute, stirring often, to toast the garlic-herb mixture.
Add the broth and mash the beans: Once the garlic-herb mixture is toasted, add the beans plus an additional sprinkling of salt and pepper. Cook for a few minutes, stirring often. Then add ½ cup of the broth mixture. Using a potato masher, mash the beans into the broth and mix around. Once thickened, add another ½ cup veggie broth. Continue to mash, mix, and stir. Once reduced, repeat this step 2 more times until all the broth is gone and continue mashing until the beans are creamy. Remove from heat and let them sit for 10 minutes to thicken.
Toast the shells: While the beans thicken, you can toast the tostada shells. Lay them in a single layer on a large baking sheet and pop them in a 350°F oven. Toast them for a few minutes until fragrant. While they cool, you can prep all of your toppings.
Enjoy: Spread a thick layer of the beans onto a tostada and stud over top your desired fresh ingredients. Finish with some flaky salt, a few dashes of hot sauce, and a squeeze of fresh lime, if desired, and enjoy!
Notes
You can store the beans in the fridge in a closed container. When reheating, you may need to add a splash of water to loosen the mixture as they will thicken quite a bit.