Chard and Mushroom Shepherd's Pie | culture: the word on cheese

Ingredients

• 1 stick (4 ounces unsalted butter, divided)
• 1 tablespoon tomato paste
• ½ medium yellow onion (chopped)
• Kosher salt (to taste)
• 4 cups chopped mixed mushrooms
• 2 to 3 Roma tomatoes (cored and chopped)
• 1 cup cooked French lentils
• 2 cups vegetable stock
• 1 bay leaf
• 3 to 4 sprigs fresh thyme
• 2 medium russet potatoes (peeled and cut into 1-inch chunks)
• 1 cup whole milk (warmed)
• 1 cup grated Caerphilly or other semi-firm (crumbly cheese, divided)
• 4 to 5 chard leaves (stems discarded, chopped into ½-inch pieces)
• 1 cup corn kernels (no need to thaw if frozen)
• 2 tablespoons red wine vinegar
• 1 tablespoon all-purpose flour