For the chimichurri vinaigrette: Mix the olive oil, vinegar, garlic, red pepper flakes, cilantro, hoja sante if using, oregano and salt and pepper to taste. Add the parsley just before serving. Makes about 3/4 cup.
For the steaks: Using tongs, roast the chiles over a stove top flame, rotating so the chiles blacken evenly on all sides, 3 to 5 minutes. (Alternatively, place on a pan under the broiler and broil a few minutes each side until blackened.) Place the chiles in a bowl and cover with plastic wrap. Let stand for 10 minutes. When cool enough to handle, peel the charred skin. Stem and seed the chiles and slice into thin strips. Set aside.
In the olive oil, slowly cook the garlic in a saucepan over medium heat, stirring occasionally, until the cloves are fragrant and have softened, about 10 minutes. When cool, cut into thin slices, or you can also leave whole.
Lay the steaks flat on a work surface. Make 3 incisions in each steak parallel to the work surface to create pockets. Stuff each with roasted chiles and garlic. Sprinkle generously with salt and pepper.
Grill the steaks over medium-high heat to the desired doneness, about 5 minutes on each side. Serve with the chimichurri vinaigrette.