Bring a large pot of salted water to a rolling boil.
Halve the onion horizontally without cutting through the root, peel it, then thinly slice using a mandolin.
In a wide pan, melt butter with oil over medium-low heat, then gently cook the onion for 10 to 15 minutes until golden and soft; add a splash of water if it starts sticking.
While onions cook, add spaghetti to the boiling water and cook until just tender.
If using scallops, heat a small pan over high heat with oil, season scallops with salt and pepper, then sear for 2 minutes per side without moving them.
Once onions are nicely browned, stir in crushed garlic and cook for 1 minute until fragrant.
Add chilli crisp, chopped capers, and the spice mix, stirring for another minute to combine flavors.
Pour in evaporated milk or cream, sprinkle in grated parmesan, stirring continuously until a rich, creamy sauce forms; reduce heat to low.
Reserve 2 cups of pasta water, then add 1 cup into the sauce to loosen it.
Drain pasta and toss into the sauce, stirring until well coated; add more pasta water if the sauce feels too thick.
Serve pasta hot, topped with scallops if desired.
