Make the Guacamole — Mash the avocados in a medium bowl until mostly smooth. Stir in lime juice, garlic, red onion, cilantro, salt, and pepper. Taste and adjust seasoning.
Top the Shells — Place tostada shells on a baking sheet. Sprinkle cheddar and Monterey Jack evenly over each shell.
Bake — Bake at 375°F (190°C) for 5–7 minutes until the cheese is fully melted and slightly bubbly.
Add Guacamole — Remove from oven and spread a generous layer of guacamole over the hot melted cheese.
Add Toppings — Top with cherry tomatoes, jalapeño slices, fresh cilantro, and a drizzle of sour cream.
Serve — Serve immediately while the shells are still crispy. Pair with salsa, hot sauce, or fresh lime wedges.