In a large cast-iron pan over medium-high heat, add about 1 tablespoon of olive oil. Add the onion and fennel seeds, stirring to coat them with oil. Sauté for about 8 minutes or until onions are charring in places, stirring occasionally. Remove from heat and transfer to a plate and set aside.
In a small bowl, combine the butter, garlic, paprika, and basil, salt and pepper to taste. Mix well to combine. Set aside.
In a small food processor, add the sun-dried tomatoes (minus the 4-6 for garnish) and the yogurt. Process to combine until smooth. Add a little water if necessary.
Return the cast-iron pan to high heat. Coat the steaks with a little olive oil and season with pepper on both sides. When the pan is very hot, place the steaks in the pan (they should sizzle) and cook for 3-4 minutes per side or until cooked to medium rare. Transfer cooked steaks to a plate and let rest for 10 minutes.
With a sharp knife, very thinly slice the steaks. Place a small pot over medium heat, add the basil and paprika butter. When the butter is melted, toss the steak slices in the butter. Remove the pot from the heat and set aside.
To serve; Spread the tomato yogurt on the toasted bread. Divide the steak slices among the toast, top with the red onion, and garnish with basil leaves and sliced sun-dried tomatoes.