The day before you plan to make the tortas, combine the heavy cream and buttermilk in a glass bowl or jar with a lid and stir or shake well to blend thoroughly. Allow to sit in a warm place, unrefrigerated, until cream is thickened and somewhat "set," up to 6 hours. Shake well and refrigerate overnight before using crema. (Or, alternatively, purchase store-bought Mexican Crema and skip this step entirely.)

In a blender or in the bowl of a food processor combine the chopped onion, garlic, 3 tablespoons of the vegetable oil, half of the lime juice, cumin, cilantro, oregano and 1 teaspoon of the salt. Process until smooth. Place the strips of flank steak in a shallow nonreactive dish and spoon the marinade over the meat. Toss to coat thoroughly and cover with plastic wrap. Refrigerate for at least 1 and up to 4 hours, turning occasionally.

While the meat is marinating, prepare the salsa.

Remove the meat from the marinade and pat dry on paper towels. Rub the meat all over with the remaining 3 tablespoons of vegetable oil and season on both sides with the remaining teaspoon of salt. Heat a large nonstick skillet over high heat and, when very hot, quickly sear the steak slices, in batches so as not to overcrowd the skillet, until browned and just cooked through, 1 to 2 minutes. Remove and allow to rest, add remaining lime juice on top.

Split the hoagie rolls in half lengthwise and divide the meat evenly between the bottoms of the rolls. Top the meat with sliced avocado, sliced tomato, and sliced jalapenos. Spread the cut sides of the tops of the bread with mayonnaise, then drizzle the crema over the bottom of each sandwich. Top with cilantro leaves, then place the sandwich tops over the filled bottoms, pressing downward to secure tops.

Heat a large skillet or griddle to medium high. When hot, brush the skillet lightly with vegetable or olive oil and lightly brush the tops of each sandwich. Place the sandwiches in the skillet, in batches if necessary, and cook, pressing downward with another skillet or with a spatula. Cook until bread is toasted and slightly crispy. Turn sandwich over and continue to cook another 1 to 2 minutes, or until bread is crispy on top. Remove from the skillet, cut each sandwich in half, and serve immediately, with the salsa on the side for guests to use at their own discretion. Garnish with lime wedges.

Preheat the broiler to high.

Place the tomatoes, onion, garlic cloves, and chiles on a baking sheet lined with aluminum foil and drizzle with the olive oil. Place in the oven about 6 inches from the broiler and cook, turning occasionally, until charred around the edges, 10 to 15 minutes. (If the chiles and garlic begin to brown before the tomatoes and onion are browned, remove them and return the baking sheet to the oven until tomatoes are done.) Transfer to a container with a tight-fitting lid, cover, and let sit for 20 minutes. Remove the seeds(optional) from the chiles and any tough skins from the tomatoes and transfer to a blender or food processor. Add the lime juice and process on high until smooth and thick. Transfer to a small nonreactive container, season with salt and pepper, to taste, and set aside until ready to serve the tortas.

Yield: About 2 cups salsa