Black Bean Mango Salsa: Combine first 7 ingredients in a large bowl. Season to taste with salt & white pepper. May be prepared and refrigerated, covered, up to 1 day ahead. Bring to room temperature 1 hour before serving.
Roasted Yellow Pepper Vinaigrette: Combine all ingredients in a food processor and process until smooth. Season with salt and pepper.
Seared Tuna: In a large saute pan over high heat, heat the oil until it begins to smoke. Season each steak with salt and pepper to taste and cook for 30 seconds or until brown and crisp. Turn the tuna over, and cook for 10 seconds more or until done to your liking. Cut the seared tuna into 1 1/2" squares just before serving.
In a medium skillet over high heat, heat the peanut oil to 375 degrees F or until an edge of a tortilla sizzles when it is immersed. Fry the tortilla until crisp, about 1 1/2 minutes on each side. Drain on paper towels and set aside. For each tostada, toss jicama in vinaigrette and place 2 tablespoons on each tortilla, top jicama with a thin layer of salsa and place a piece of grilled tuna on top. Garnish with a cilantro leaf and yellow pepper vinaigrette.