Stuffed Portobello mushrooms with blue cheese

Name: Stuffed Portobello mushrooms with blue cheese
Ingredients: 2 portobello mushrooms, stalks removed and chopped; 50g/1¾oz unsalted butter; 2 sprigs fresh thyme; 1 tbsp olive oil; ½ onion, finely sliced; 2 garlic cloves, finely chopped; ½ pepper chopped; 50g/1¾oz mild vegetarian blue cheese; 50g/1¾oz white breadcrumbs; 50g/1¾oz fresh chives; salt and freshly ground black pepper