Preheat oven to 425° with shelves on the top and bottom racks. Heat a large pot of salted water over high heat. In a small bowl combine 1 tbsp olive oil, zest of lemon, juice of half the lemon, oregano, half of the garlic, 1/2 salt, and 1/4 tsp pepper. Toss with shrimp and set aside.
When water boils add macaroni and cook per instructions for al dente. Remove heat, drain, and return to pot.
Heat a large cast-iron skillet over medium heat. Add butter to the pan and when melted stir in saffron. Add chorizo, red onion, light-green/white parts of scallions, bell pepper, remaining garlic, and 1/2 tsp salt. Cook, stirring occasionally, for 4-5 minutes until starting to soften.
Add flour and stir to combine, cook for 1 minute while stirring. Stir in milk and cream. Bring to a simmer then mix in Manchego cheese and white pepper. Remove from heat.
Stir macaroni into sauce. Bake on bottom rack for about 20 minutes until bubbling and browned around the edges.
Meanwhile, add mushrooms to a sheet pan and toss with 1 tbsp olive oil, juice from half lemon, and a pinch of salt and pepper. Bake on top rack for 12 minutes then flip and add shrimp to the sheet pan. Return to top rack for 5-6 minutes until shrimp is cooked through.
Serve macaroni topped with mushrooms, shrimp, and green parts of sliced scallions.