Add tomatoes, onions, jalapeno peppers, and cilantro to a medium bowl.
Add lime juice and season with salt.
Mix, adjust salt to taste and set aside.
Make the guacamole spread.
Add avocado pulp to a bowl and mash it with a fork or a potato masher.
Add diced onion, garlic, lime juice, and salt.
Mix nicely to combine, adjust salt to taste, and set aside.
Assemble
Spread a spoonful of guacamole on a tostada.
Add some black beans, lettuce, and fresh salsa on top.
Sprinkle queso fresco and drizzle with Mexican crema.
Serve with your favorite salsa (read notes).
Notes
If you can't find store-bought tostada shells where you live, make your own at home. Read the post section "how to make corn tostadas" for instructions.
You can use homemade salsa or store-bought, take a look at our favorites recipes that go perfectly with tostaguac: