Bring a large pot of salted water to a boil. Wash and dry all produce. Trim and dice zucchini into 1/4-inch pieces. Dice mushrooms into 1/4-inch pieces. Dice tomato into 1/4-inch pieces. Halve, peel, and finely dice onion.

Once water is boiling, add spaghetti to pot and cook until al dente, 9-11 minutes. Reserve 2 cups pasta cooking water, then drain.

Meanwhile, heat a large drizzle of olive oil in a large pan over medium-high heat. Add onion and cook until lightly browned, 3-4 minutes. Add mushrooms and zucchini and cook until lightly browned and slightly softened, another 5-7 minutes. Season generously with salt and pepper.

Add tomato, tomato paste, Italian Seasoning, 3/4 tsp garlic powder (11/2 tsp for 4 servings), and a small pinch of chili flakes to pan. Cook, stirring, until thoroughly combined, 1-2 minutes. Add stock concentrate and 11/2 cups reserved pasta cooking water. Bring to a boil, then reduce heat to a simmer. Cook until thickened and veggies are soft, 7-8 minutes.

Meanwhile, place 2 TBSP butter (4 TBSP for 4 servings) in a small bowl and microwave until just softened, about 10 seconds (do not melt). Stir in 1 TBSP Parmesan (2 TBSP for 4 servings), remaining garlic powder, a pinch of salt, and a pinch of chili flakes (to taste). Halve demi-baguette and toast until golden. Spread butter mixture onto cut sides of baguette, then halve on a diagonal.

Add spaghetti, 1 TBSP butter (2 TBSP for 4 servings), and a splash of remaining pasta cooking water to sauce. Toss to combine, adding more pasta water as needed for sauce to fully coat pasta. Taste and season generously with salt and pepper. Divide finished pasta between bowls. Top with remaining Parmesan and a pinch of remaining chili flakes, if desired. Serve with garlic bread on the side.