Heat a large skillet over medium-high heat with a splash of oil. Add the onion and cook for 4 minutes until softened and just starting to brown at the edges. Stir in the garlic and tomato paste, cooking for 1 minute until the paste darkens slightly and smells sweet. This step builds a flavor base that slow cooking alone can't achieve.

Transfer the onion mixture to your slow cooker. Add the potatoes, carrots, mushrooms, vegetable broth, thyme, smoked paprika, and bay leaf. Stir to combine, making sure the potatoes are mostly submerged. The broth will look thin now but concentrates beautifully over the long cook.

Cover and cook on LOW for 7 to 8 hours or HIGH for 4 hours, until the potatoes and carrots are fork-tender but not falling apart. Avoid lifting the lid during cooking - it adds 15 to 20 minutes each time.

Remove the bay leaf. Stir in the frozen peas and let them warm through for 5 minutes. The residual heat is enough - cooking longer turns them gray and mushy. Taste and adjust salt and pepper; you'll likely need more salt than expected since potatoes absorb a lot.

Ladle into bowls and top with fresh parsley. Serve with crusty bread for soaking up the broth, or over buttered egg noodles if you want something more substantial.