Cut mushrooms into 1/3" slices. Peel and halve shallot. Cut into 1/4" slices. Carefully ladle out 1/2 cup boiling water and add to a small mixing bowl with sun-dried tomatoes. Keep water boiling. Set aside to rehydrate.

Place a large non-stick pan over medium heat. Add pine nuts to dry, hot pan and toast while stirring until golden brown, 1-2 minutes. Transfer pine nuts to a plate. Reserve pan; no need to wipe clean.

Pour polenta in a steady stream into remaining boiling water while whisking. Continue whisking over medium-high heat until polenta comes to a boil, 1-2 minutes. Whisk Parmesan cheese (reserve a pinch for garnish), half the butter (reserving remaining for sauté), 1/2 tsp. salt, and a pinch of pepper until fully incorporated. Remove from burner. Taste and add more salt if desired. Polenta should be thick and creamy. Cover and set aside.

Return pan used to toast pine nuts to medium-high heat. Add 1 Tbsp. olive oil and mushrooms to hot pan. Cook, stirring occasionally, until browned and tender, 3-5 minutes. Add shallots and cook, stirring occasionally, until slightly softened, 1-2 minutes.

Add sherry to pan and cook until mostly reduced, 2-3 minutes. Add sun-dried tomatoes and rehydrating liquid and cook until mostly reduced, 3-5 minutes. Remove from burner and add spinach, 1/2 tsp. salt, 1/4 tsp. pepper, and remaining butter. Stir together until butter is incorporated and spinach is wilted.

If polenta gets too firm while sitting, whisk in water 2 Tbsp. at a time over medium heat to loosen. Place polenta on a plate or in a shallow bowl. Top with mushroom and sun-dried tomato sauté and sprinkle with pine nuts. Garnish with reserved Parmesan.