Heat olive oil in a pan and add the garlic and spring onions and sauté until they turn fragrant. Add the flour and mix well. Drizzle the milk slowly into the pan and stir constantly to avoid any lumps. Using a whisk at this stage would be ideal. Add the mustard paste and mix well. Season with salt and pepper. Once the mixture starts to thicken, add in the cream and stir until the sauce reaches the consistency of a thin crepe batter. Add the butter right at the end and mix it in until the sauce is glossy and rich. Take off the heat and add the lime juice. Check for seasoning and adjust if needed.