For the garlic croutons: Preheat the oven to 350 degrees F. Line a baking sheet with parchment.
Add the bread cubes to the lined baking sheet and reserve. Add the olive oil, butter and garlic to a small saucepot and cook over medium-low heat until the butter is melted, stirring to combine. Remove the garlic cloves, then drizzle the butter mixture over the bread, tossing to get everything coated. Bake until golden brown and crisp, about 15 minutes. Let cool.
For the soup: Put the lentils, carrots, celery and onion into a stockpot or Dutch oven. Pour in the vegetable stock. Add the thyme, bay leaf and 1 teaspoon black pepper and stir to combine. Bring to a rapid simmer, then cover partially and cook on medium low until the lentils are tender, 30 to 45 minutes.
Remove the bay leaf and thyme stems. Add the marinara sauce and salsa and stir to combine. Add the peas and parsley. Season with salt and pepper and stir to combine. Cook for an additional couple of minutes to warm the peas through.
Ladle the soup into serving bowls. Top each bowl with a spoonful of sour cream, a sprinkle of whole parsley leaves, a drizzle of extra-virgin olive oil and a couple of croutons. Store and refrigerate in airtight containers or freeze.