Preheat your oven to 400°F. While it heats, cut the baguette into 1-inch cubes and mince the 3 garlic cloves for the croutons (keep separate from the 1 garlic clove for the dressing). Toss the bread cubes with 2 tablespoons olive oil, minced garlic, salt, and black pepper, then spread on a baking sheet and bake for 8-10 minutes until golden and crispy. Remove from oven and set aside in a bowl. I like to make croutons first so they're completely cool and crunchy before tossing with the salad.
In a bowl, combine the mayonnaise, Greek yogurt, freshly squeezed lemon juice, Dijon mustard, Worcestershire sauce, minced anchovy fillet, the remaining 1 minced garlic clove, and black pepper. Whisk together until smooth and creamy—the anchovy will dissolve into the dressing, adding depth of flavor without any fishy taste. Set the majority of this dressing aside for tossing the salad. In a small separate bowl, mix 1/4 cup of this prepared dressing with 1/4 cup grated Parmesan to create the topping for the salmon.
Pat the salmon fillets dry with paper towels and bring them to room temperature if needed. Season each fillet generously with salt and black pepper on both sides. Spread the Parmesan dressing mixture (from Step 2) evenly over the top of each salmon fillet, then sprinkle with the additional 1/4 cup grated Parmesan, pressing gently so it adheres to the dressing. Arrange the fillets on a baking sheet and bake at 400°F for 12 minutes, until the salmon is cooked through and the Parmesan crust is golden. I find that using Parmigiano-Reggiano gives a much richer, nuttier flavor to the crust compared to regular Parmesan.
While the salmon finishes cooking, chop the romaine lettuce into bite-sized pieces and thinly slice the red onion. In a large bowl, toss the lettuce, croutons from Step 1, and red onion with the reserved Caesar dressing (from Step 2), making sure everything is evenly coated. Divide the dressed salad among serving plates. Once the salmon is finished baking and the Parmesan crust is golden, carefully place one fillet on top of each salad. Serve immediately while the salmon is still warm and the croutons are crispy.