Set a large, 1-gallon stock pot with a pasta insert 3/4 full with water over high heat and bring to a boil. Add 1/4 cup of the salt to the water as it boils. Place the linguine in the salted water and return to a boil, stirring occasionally. Cook the pasta until nearly al dente, 10 to 12 minutes. Drain and set aside, reserving 1 cup of the pasta cooking water.
While the pasta cooks, set a 14-inch saute pan over medium-high heat and add the olive oil and butter to the pan and, once the butter melts, add the onions. Cook, stirring occasionally, until the onions have softened and caramelized, 10 to 12 minutes. Add the garlic and anchovies to the pan, stirring until fragrant and the anchovies have melted, 2 to 3 minutes. Add the drained pasta and the reserved pasta cooking water to the pan and season with the remaining 1 1/2 teaspoons kosher salt, pepper, and parsley. Cook, tossing to combine, until the pasta is heated through and the water is nearly evaporated, 3 to 4 minutes. Remove from the heat, sprinkle with the toasted bread crumbs and toss to combine. Serve while hot.