| 12 | medium-size | Mushroom caps |
| 1 | tbsp | Olive oil |
| 1 | tbsp | Sweet butter |
| 1/2 | C | Finely chopped yellow onion |
| 2 | tbsp | Coarsely chopped walnuts |
| 1 | Garlic clove, peeled and minced | |
| 5 | oz | Frozen spinach, thoroughly defrosted and squeezed dry |
| 1 | oz | Feta cheese, crumbled |
| 1 | oz | Gruyere cheese, crumbled |
| 2 | tbsp | Minced fresh dill |
| Salt and freshly ground black pepper to taste |