In a large non-stick skillet, melt 2 Tbsp butter over medium-high heat. Add mushrooms, season with salt and pepper. Sautee mushrooms until water releases. Add umami seasoning and dried tomatoes, cooking a few minutes more so that tomatoes absorb some of the liquid. Remove mushrooms and tomatoes to a separate bowl and set aside.
Fill a large pot with salted water, bring to boil and cook pasta according to package directions. When ready, drain water and set pasta side, covered.
While the water is coming to a boil, melt the remaining 3 Tbsp butter over medium-high heat in the same pan used to cook the mushrooms. Once melted, add onion and season with salt and pepper. Cook onions until translucent, about 5 minutes.
Whisk in flour. Cook, stirring constantly until mixture is pale golden and has a slightly nutty aroma, about 2 minutes. Whisking constantly, add milk in 1 cup increments. Be sure milk is fully incorporated before adding next batch. Keep whisking until mixture comes to a boil, then reduce heat to low. Simmer, stirring occasionally until sauce thickens, about 10 to 15 minutes more.
Add spinach, cooking until wilted, approximately 3 minutes more. Add cheese and stir until melted.
In a large serving bowl, stir together pasta, cheese sauce and mushroom/tomato mixture. Top with white truffle oil and stir to combine.