Preheat oven to 450 degrees F. Coat peppers with olive oil and place on a foil lined baking sheet. Salt liberally. Bake in the oven, turning every 10 minutes until peppers are blistered and black, about 20 to 30 minutes total.
Meanwhile, add 2 tablespoons of olive oil to a hot saute pan. Add the serrano chiles and the garlic. Lower the heat to moderate, and cook, turning occasionally, until the chiles are softened, lightly browned, and blistered on all sides. Remove the pan from the heat and let the chiles cool in the oil for several minutes
When the roasted bell peppers are blistered and blackened, removed them from the oven, place in a glass bowl and cover with plastic wrap. This will steam the skins off and make them easier to remove. After 15 to 20 minutes, remove the peppers and peel. Remove the seeds, stems and ribs. Do not run under water – this will just wash the flavor off! You should have about 4 cups.
Note: You can roast the peppers under the broiler or over a gas flame, if you prefer, but I like to do them in a hot oven. It takes a little longer, but you don't have to monitor them as closely.
Chop the garlic finely, then use the side of your knife to mash them to a paste – a pinch of grey salt will help this process. Mashing insures that the garlic add just an undercurrent of flavor, with no big pieces to bite into. Add to the blender.
When cool enough to handle, peel the chiles and remove the stems and seeds (or leave some or all of the seeds if you prefer a spicy sauce). Then add the tomato puree and the oregano (no need to chop the oregano because it's going to be blended in). Add to the blender the olive oil used to roast the chilis and garlic. Turn the blender on high until everything is smooth and thick.
Turn a medium saute pan on high for 30 seconds with about 1/4 cup of olive oil. Stand back as you pour the blender mixture into the pan. Be careful, it can splatter. This second heating brings the flavors together. Season with salt and pepper. Whisk in the remaining 1/4 cup olive oil and the red wine vinegar. The vinegar gives it a little sharpness and protects it in the refrigerator for about a week.
Ladle it into canning jars and store in the refrigerator for a week. It can be frozen for 5 months.
Preheat a grill to high. Season the steak with salt and pepper and drizzle with olive oil. Grill the steak to medium-rare, about 4 minutes per side. Remove from the grill and let rest for 10 minutes.
To a large pot of boiling, salted water, add the spaghettini and cook to al dente. Drain and toss with the salsa rossa.
Slice the flank steak thinly against the grain and place on top of the pasta.