Roast the Portobello Mushrooms: Preheat the oven to 450 degrees Fahrenheit with a rack in the center position. 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. Season with salt and pepper. Roast for 15 to 25 minutes (*this time will vary depending on the size of your mushrooms, check after 10 minutes or so to be sure), or until tender. If any moisture collects in the mushroom caps, carefully drain and discard. Set aside on a large plate as you prepare the filling and crispy panko breadcrumb topping.
Prepare the Sautéed Spinach: Heat the 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 the butter in a small sauté pan over medium heat. Once the butter is just beginning to sizzle, add the chopped shallots and a pinch of salt and cook for 1 to 2 minutes, stirring continuously. Add the panko breadcrumbs and minced garlic, and toast for three to four minutes, stirring constantly to prevent the breadcrumbs from burning, or until the breadcrumbs are very light golden in color. Transfer to a shallow bow and season to taste with salt and pepper.
Assembly: Reduce the oven temperature to 425 degrees Fahrenheit. Line the same sheet pan with parchment paper (*note: if you have a baking rack that can be set within a baking sheet, I prefer to place the mushrooms on this, as it helps prevent the mushrooms from sitting in any juice that releases during cooking).
Distribute the roasted mushrooms, stem side up, on the sheet pan, in the center of the sheet pan so that they are just touching each other. 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 1/2″ thick slices (roughly half an ounce). It may crumble a bit, but do your best to keep it in a roughly round slice. 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 degrees 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 brown. Serve immediately.