Step 1Heat the oven to 375 degrees.
Step 2In a large bowl, toss the pumpkin and onions with 3 tablespoons olive oil, one-half teaspoon salt, several grinds of pepper, the rosemary and thyme. Place the vegetables on a foil-lined baking pan and roast until softened and lightly caramelized, about 45 minutes, tossing every 15 minutes for even coloring. Remove the pan and allow the pumpkin and onions to cool to room temperature.
Step 3Meanwhile, in a large skillet over medium heat, cook the bacon until it is crisp, 10 to 12 minutes. Remove the pan from the heat and remove the bacon to a paper towel-lined plate to cool. Drain all but 3 tablespoons of the bacon fat, discarding or reserving the rest for another use.
Step 4Add the leek strips to the pan and place over medium-high heat. Cook, stirring frequently, until they soften and just begin to color, about 5 minutes. Stir in the garlic and cook until aromatic, about 30 seconds. Stir in the white wine and cook, scraping any of the flavorful bits that stick to the bottom of the pan, until the wine evaporates, 2 to 3 minutes. Remove from heat.
Step 5Place the leek strips and any cooking liquid in a medium bowl. Whisk in the vinegar and mustard. Drizzle in the walnut oil while whisking to emulsify. Season with one-eighth teaspoon salt and a few grinds of pepper, or to taste. Set aside in a warm place.
Step 6Toss the walnuts with 3 tablespoons olive oil and one-half teaspoon salt. Spread on a sheet pan and toast in the 375-degree oven until the nuts are fragrant and a rich brown color, 6 to 8 minutes. Remove and cool slightly. Set aside. The salad components can be made up to this point 1 day in advance. Refrigerate the components separately, and warm the dressing before folding into the salad.
Step 7In a large serving bowl, toss the greens with two-thirds of the vinaigrette. Gently fold in the roasted pumpkin and onions, the bacon and toasted walnuts. Add extra vinaigrette as needed to lightly coat the contents.
Step 8Serve immediately, or cover and chill until needed, up to several hours in advance. Allow to warm slightly before serving.