Bring a large pot of salted water to a boil. Wash and dry produce.

Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Peel and mince or grate garlic. Roughly chop parsley.

Once water is boiling, add spaghetti to pot. Cook, stirring occasionally, until al dente, 9-11 minutes.

Reserve 1½ cups pasta cooking water (2 cups for 4 servings), then drain.

Heat a large drizzle of oil in a large pan over medium-high heat. Add mushrooms, a generous pinch of salt, and pepper. Cook, stirring occasionally, until mushrooms are tender, 4-5 minutes.

Add garlic and as many chili flakes as you like; cook until slightly browned and fragrant, 1 minute more. Remove from heat.

Grate Parmesan over a second small bowl until you have 2 TBSP (4 TBSP for 4 servings). Reserve for serving. 

Grate remaining Parmesan over bowl with egg mixture. Add paprika, stock concentrate, a pinch of salt, and pepper. Whisk to combine.

Slowly whisk in ⅓ cup reserved pasta cooking water (⅔ cup for 4).

To pan with mushrooms, add drained spaghetti and 1 TBSP reserved pasta cooking water (2 TBSP for 4 servings). Toss to combine.

Return pan to medium-low heat. Stir in egg mixture and half the parsley. Cook, stirring constantly, adding splashes of reserved pasta cooking water as needed, until pasta is fully coated and sauce is creamy and slightly thickened, 2-3 minutes.

Turn off heat. Taste and season with salt and pepper.

Divide pasta between shallow bowls. Top with remaining parsley and reserved Parmesan. Serve.