Mushroom Patty Melt

Bahan-bahan (4 porsi)

• ½ cup vegetable oil (divided, $0.33)
• 1 colossal onion (thinly sliced (565g, about 3 cups) $0.93*)
• 4 large portobello mushrooms ($5.36)
• 2 g arlic cloves (minced, (1 Tbsp) $0.12)
• ¼ cup balsamic vinegar ($0.48)
• ⅛ cangkir kecap ($0,11)
• ½ tsp dried thyme ($0.22)
• 8 slices roti gandum ($1,37)
• ⅓ cup mayonnaise ($0.30**)
• 8 slices Swiss cheese ($1.31**)
• ¼ tsp salt ($0.01)
• 4 Tbsp butter (divided, $0.50***)