Bring 1 ¾ cups of water, the fennel seed, dried marjoram, and 1 tablespoon of olive oil to a boil in a pot over medium-high heat. Add in the wild rice, cover, and turn to low heat. Cook for 45 minutes, adding a bit more water if needed to finish cooking. Turn off the heat and let sit (still covered) for 10 minutes, then uncover and fluff the rice with a fork. Mix in ½ teaspoon of salt.
Heat 2 tablespoons of olive oil in a sauté pan over medium heat. Add in the leek and garlic and cook, stirring often, for 4 minutes. Add in the mushrooms, 1 teaspoon of thyme, ¼ teaspoon of salt and freshly ground black pepper, and cook for 5 minutes, stirring occasionally.
Combine the mushroom mixture with the wild rice, along with 4 teaspoons of sherry vinegar, the chopped parsley, and toasted slivered almonds. Taste and season with additional salt and pepper, if necessary. The wild rice stuffing can be made 2 days ahead of time and kept refrigerated. If doing so, let the stuffing come to room temperature before using.