Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the onion and cook until softened and just starting to turn golden at the edges, about 6 minutes. Stir in the garlic and tomato paste; cook for 1 minute until fragrant and the paste darkens slightly.

Pour in the red wine (if using) and scrape up any browned bits from the pan. Let bubble for 30 seconds, then transfer everything to your slow cooker. Add the remaining 1 tablespoon olive oil to the same skillet, increase heat to medium-high, and sear the mushrooms until they release their liquid and turn golden in spots, about 5 minutes. Add to the slow cooker.

Add the potatoes, carrots, celery, thyme, rosemary, bay leaf, and vegetable broth to the slow cooker. Stir to combine, nestling the vegetables into the liquid. Season with 1 teaspoon salt and 1/2 teaspoon pepper.

Cover and cook on LOW for 7 to 8 hours or HIGH for 4 to 5 hours, until the potatoes and carrots are tender when pierced with a fork but still hold their shape. The broth should have reduced slightly and taken on a rich, earthy color.

Stir in the frozen peas during the last 10 minutes of cooking. Remove the bay leaf, taste and adjust seasoning with more salt and pepper as needed. Ladle into bowls and sprinkle with fresh parsley. Serve with crusty bread for soaking up the broth.