Preheat the oven to 400°F.
Arrange the Portobello caps on a work surface. Remove the stems, and brush both sides of each cap with olive oil. Season with salt and pepper. Bake the mushrooms (trimmed / belly-side up) on a parchment-lined baking sheet for 15 minutes, until browned and tender. Remove and set aside.
Meanwhile, arrange the slices of bread on a second baking sheet and transfer to the oven. Bake until lightly browned and toasty, 5 minutes. Remove and set aside until cook enough to touch. Then finely chop into cubes.
In the meantime, warm the plant-based butter in a large skillet over medium heat. Add the leek and fennel and sauté until translucent, about 7 minutes. Add the garlic and sage, and cook for another 2 minutes. Deglaze the pan with the broth and set aside.
Divide the stuffing between the caps, pressing down to create a compact mound. Turn the oven temperature down to 350°F, and return the pan to the oven. Bake for 15-20 minutes, until the top of the stuffing is crusty and brown.
Sprinkle with chives and serve.