In a hot wok, heat cooking oil over medium heat. Add garlic, ginger, most of the red chilli and green onion slices (reserve a bit of both, for garnish). Cook, stirring, until fragrant and just beginning to soften, 30-45 seconds. Add sauce and cook until warmed. Add udon noodles. Stir to combine. Cook, adding additional 1-2 Tbsp. hot water as needed, to loosen the sauce so it coats the noodles well.