For soup: Heat butter over medium heat in a large pot. Add leek and cook until soft, about 5 min. Stir in garlic, fenugreek seeds, and salt. Add apple and remaining veggies and cook for 5 more min., stirring frequently. Add broth, bring to a boil, then reduce heat and simmer, uncovered, for 30 to 40 min., or until veggies break apart easily when pierced with a fork. In batches, transfer soup to a blender and puree until smooth. Transfer back to pot, add maple syrup (if using) season with pepper to taste. Keep over low heat until ready to serve.
For mushrooms: While soup simmers, melt 3 Tbsp. of the butter over medium-high heat in a wide skillet. Add mushrooms and cook, stirring often, for about 15 min. until mushrooms are well browned. Meanwhile, place fennel seeds in a small zip top bag and seal. Using a meat mallet or back of a heavy spoon, crush seeds into a coarse texture. Add remaining 1 Tbsp. butter to skillet with mushrooms. Once butter melts, stir in crushed fennel seeds and garlic and cook 1 more min., stirring constantly. Remove skillet from heat and season mushrooms with salt & pepper to taste.
To serve: Fill bowls with soup and top with a generous scoop of mushrooms. Garnish with reserved fennel fronds.