In a small blender or food processor, combine dressing ingredients. Process until smooth, adding a little water, if necessary, for thinner consistency. Set aside. This makes about ¾ cup. You will only need 5 tablespoons for this recipe. Refrigerate remaining dressing in an airtight container for up to a week.
Add butter and oil to a large skillet over medium heat. When butter is melted, add garlic and sauté 30 seconds. Add shrimp and season with 1/8 teaspoon salt and pepper. Cook 2 minutes, flip, then add another 1/8 teaspoon salt and pepper. Cook an additional 2 minutes or until shrimp are opaque and cooked through. Remove from heat and add red pepper (if using), lemon juice, and parsley. Toss to evenly coat.
Meanwhile, heat tortillas according to package directions. In a medium bowl, toss lettuce with 2 tablespoons of Caesar dressing.
Divide tortillas and shrimp evenly among 4 plates. Spoon pan sauce over shrimp, top each with freshly grated parmesan, scallions, and Caesar slaw. Divide remaining 3 tablespoons of dressing evenly over tacos.