Combine sour cream and goat cheese in a bowl, stirring until blended. Set aside.
Sprinkle tuna steaks with salt and pepper. Heat oil in a grill pan or skillet over medium-high heat. Add tuna, and cook 2 minutes per side or until desired degree of doneness. Remove from skillet, and set aside.
Add tomatoes to skillet, and remove from heat. Cook tomatoes with residual heat for 1 minute, stirring constantly. Add spinach; stir 1 minute or until slightly wilted. Remove from skillet.
Warm tortillas according to package directions. Slice tuna across the grain into 1/4-inch strips. Place strips on half of each tortilla. Top with spinach and tomatoes, and drizzle with goat cheese mixture. Fold tortilla, and serve immediately.