Drizzle a tiny bit of oil in a saucepan over medium heat. Add onion, garlic, salt & pepper & cook until tender-crisp, about 3-4 minutes. Add flour & whisk until well combined with onions & garlic. Pour the milk slowly while whisking constantly, making short pauses from time to time to whisk until sauce becomes nice & smooth, then start pouring again. Once the milk has been added, whisk in the Dijon mustard, basil paste, dried dill & parsley. Stir in clams & set aside.