Place a 12 inch skillet or a Dutch oven over medium-high heat. Drizzle in the olive oil, then add the onions and salt and stir to coat with oil. When the onions start to sizzle, reduce heat to medium-low. Stir every 10-15 minutes, scraping the bottom of the pan, and if they start to stick, reduce to low. Keep cooking for an hour, they will shrink and turn the color of caramel candy. When you have 3 cups or so, take off the heat. Let cool. Measure 1 cup for the pasta, store the rest for other dishes.
While the onions cook, roast the Brussels sprouts. Preheat the oven to 400 F. Spread the halved sprouts on a sheet pan and drizzle with oil, sprinkle with smoked salt. Roast for 20 minutes. Let cool. Reserve 1 1/2 cups for pasta, store the rest for other meals.
For pasta, bring a big pot of water to a boil, add salt. Cook pasta according to package directions, about 12 minutes. Add the carrots for the last two minutes. Drain well.
In the pasta pot, place the cup of caramelized onions and brussels sprouts, and a splash of olive oil just to keep it moving as you stir over medium heat. When heated, add the pasta and toss to mix, stir until all is hot again.
Serve sprinkled with pistachios.