Warm the oil in a frying pan over medium-high heat. Stir in the onion and cook until nearly tender. Stir in the garlic and cook for a few seconds until fragrant. Pour in peas and stir to thaw. Pour in the vegetable stock and evaporated milk. Turn heat to low, cover and cook for 5 minutes. Once the peas have cooked, pour them into a blender and puree until smooth.
Pour the pureed sauce back into the frying pan and season to taste with salt and pepper. Simmer over low heat until the sauce reaches desired consistency.