Fill a large pot with lightly salted water and bring to a rolling boil. Stir in fettuccine, bring back to a boil, and cook over medium heat until tender yet firm to the bite, about 8 minutes. Drain.
Heat oil in a large saucepan over high heat. Add onion and garlic; cook, stirring constantly, until slightly browned, 2 to 4 minutes. Add 1/2 cup coconut milk, oyster mushrooms, red bell pepper, and green bell pepper. Cook and stir until just tender, 3 to 5 minutes.
Stir fettuccine and remaining 1/2 cup coconut milk into the saucepan. Season with salt and pepper. Stir in spring onions and basil. Cook uncovered until sauce is creamy and coats fettuccine, 2 to 3 minutes more.