Roast the Portobello Mushrooms: Preheat the oven to 450°F (230°C). Lightly spray or brush a standard sheet pan with olive oil. Distribute the portobello mushrooms, stem side up, on the baking sheet and brush or spray lightly with olive oil. Roast for 15 to 25 minutes.
Prepare the Sautéed Spinach: Heat oil in a large sauté pan over medium-low heat. Add the sliced shallots and sauté for 3 to 4 minutes, stirring frequently, or until soft and translucent. Add the baby spinach, increase the heat to medium-high. Sauté until just wilted, stirring continuously. Season with salt and pepper and set aside.
Prepare the Crispy Breadcrumb Topping: Melt butter in a small sauté pan over medium heat. Once the butter is just beginning to sizzle, add the chopped shallots. Add the panko breadcrumbs and minced garlic, and toast, until the breadcrumbs are very light golden in color
Assembly: Reduce the oven temperature to 425°F (220°C). Line the same sheet pan with parchment paper. Distribute the roasted mushrooms, stem side up, on the sheet pan. Fill each mushroom with a large spoonful of marinara sauce. Distribute the sautéed spinach evenly among the mushrooms, spooning it on top of the sauce.
Slice the goat cheese into ½-inch thick slices. Carefully press the goat cheese slices, on both sides, in the panko breadcrumb topping and place on top of each mushroom. Sprinkle the mushrooms with the remaining breadcrumb mixture.
Bake at 425°F (220°C) for 10 to 15 minutes or until heated through and the breadcrumbs are golden brown and goat cheese is just beginning to soften. Be careful to ensure that the breadcrumbs do not burn. Serve immediately.