Preheat the oven to 425 degrees.
Cut the onions in half, slice them crosswise in ½-inch-thick half-rounds, and place them in a large bowl. Cut the fennel bulb in half lengthwise, cut the halves in 1-inch-thick wedges through the core (to keep the wedges intact) and add to the bowl. Cut the peppers in 1-inch-wide strips, discarding the stems and seeds, and add to the bowl. Toss with 4 tablespoons olive oil, the garlic, thyme, fennel seeds, 2 teaspoons salt, and 1 teaspoon black pepper. Transfer the vegetables to a very large (14 × 18-inch) roasting pan (or two sheet pans), spread them evenly in a single layer, and roast for 20 minutes, tossing once with a large metal spatula.
Meanwhile, cut the kielbasa crosswise in ¼-inch-thick slices, being careful to cut only two thirds of the way through. When the vegetables are ready, toss them again, discard the thyme branches, and place the kielbasa on top. Combine the mustard and honey and brush the top and sides of the kielbasa. Roast for 30 minutes, brushing the kielbasa with the honey mustard every 10 minutes, until it is crispy and browned. (Reserve extra honey mustard for serving.) Sprinkle the vegetables with salt and serve hot with the reserved honey mustard on the side.
Two-fer: Add leftover sausage to the Tuscan White Bean Soup or any bean, lentil, or split pea soup.
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