Preheat the oven to 400°F.
Make the taco “meat”: In a large nonstick sauté pan, cook the onion over medium-high heat until just softened, 1 to 2 minutes. Add the lentils, taco seasoning, and 2 tablespoons water and mix well. Remove the pan from the heat once the lentils are hot and set aside.
Assemble the tostadas: Arrange the corn tortillas on a baking sheet and toast in the oven until crisp, 8 minutes. Remove from the oven and layer the toasted tortillas with the lentil mixture, refried beans, cheese sauce, sour cream, tomatoes, lettuce, and hot sauce, if desired.
Heat a large nonstick sauté pan over medium-high heat. Place a tostada in the center of a flour tortilla and fold the edges of the flour tortilla toward the center to completely enclose the tostada. Carefully place the wrapped tostada in the pan, folded-side down, and sear for 3 to 5 minutes, until golden brown. Flip and cook for 3 to 5 minutes on the second side, then transfer to a plate. Repeat with the remaining tostada and flour tortilla.
Serve with additional sour cream and hot sauce, if desired.