1. Preheat your oven to 350°F or 175°C.

2. Peel the onion and chop it finely, then sauté it in butter over a gentle heat until the onion has softened and lost its raw edge.

3. Add the chopped parsley and minced garlic to the onion mixture and cook for a further 1-2 minutes, stirring occasionally to prevent burning.

4. In a large mixing bowl, combine the cooked onion mixture with the softened butter, blending the two together until they are well incorporated and smooth.

5. Shape the mixture into a neat log and wrap it tightly in plastic wrap, making sure to seal the ends securely. Refrigerate for at least 30 minutes or until the mixture has chilled and firmed up.

6. Once the mixture has chilled, slice it into thick rounds, about 1 inch or 2.5 cm in thickness.

7. Place the slices on a baking sheet lined with parchment paper, leaving a little space between each round to allow for even cooking. Bake in the preheated oven for 15-20 minutes, or until the slices are lightly golden and crispy on the outside.