• In a small saucepan, heat 1 Tbs. olive oil over medium high heat until shimmering. Add the lemongrass and ginger and cook, stirring often, until browned in spots, about 30 minutes. Add the garlic and continue to cook about 15 seconds. Remove the pan from the heat and whisk in the remaining olive oil, the orange juice and zest, lime juice and zest, and 1/2 tsp. salt. Let the marinade cool to room temperature.
• In a small bowl, mix 1/4 cup of the marinade and the sour cream; cover and refrigerate until ready to use. Transfer the remaining marinade to a bowl. Add the shrimp, cover, and refrigerate for 2 to 3 hours.
• Prepare a medium-high (400°F to 475°F) charcoal or gas grill fire. In a small saucepan, heat 1 Tbs. olive oil over medium-high heat. Add the beans, chile powder, and 1/4 tsp. salt and cook, stirring often, until heated through. Remove the pan from the heat and coarsely mash beans with a fork.
• Brush both sides of the tortillas with olive oil. Grill the tortillas for 1 to 2 minutes on each side, or until slightly charred. Remove to platter.
• Thread the shrimp onto metal skewers and grill, turning once, until the shrimp is opaque, about 4 minutes. Remove the shrimp from the skewers and slice in half lengthwise.
• Arrange 2 tortillas on each of 4 large plates. Spread the bean mixture over the tortillas and sprinkle with onion. Top with the shrimp, spoon the cream mixture over the tostadas, sprinkle each with cilantro, and serve.

Tip: These tostadas can also be cut into quarters and served as an appetizer.