To make the chimichurri, in a small bowl, whisk together 4 tablespoons oil, the vinegar, cilantro, garlic, pepper flakes and ¼ teaspoon each salt and black pepper; set aside.
Brush 3 tablespoons of the remaining oil onto the insides of the bread. Heat a 12-inch skillet over medium until droplets of water flicked onto the surface sizzle in a few seconds. Toast one bread section at a time, cut side down, until lightly golden, about 1 minute; remove from the pan and set aside.
In the same skillet over medium, heat the remaining 1 tablespoon oil until shimmering. Cook the sausages, cut sides down, until browned, about 4 minutes. Flip the sausages and cook, turning and flipping as needed, until well browned on both sides and the centers reach 160°F, another 4 to 6 minutes.
Place a sausage on each piece of baguette. Stir the chimichurri to recombine, then spoon it onto the sausages. Close the sandwiches and serve.
Optional garnish: Thinly sliced avocado OR sliced tomato OR mayonnaise OR a combination
