Heat the 1/3 cup oil in a large skillet over medium heat. Add the bread cubes and fry, tossing and stirring, until crisp and golden on all sides. Drain on paper towels.

Heat the 2 tablespoons oil in a wide soup pot over medium heat. Add the leeks, season lightly with salt, and cook gently until the leeks have softened and just begin to turn golden, 8 to 10 min. Add the parsnips and continue to cook, stirring occasionally, until the parsnips are fragrant, about 2 minutes. Add the sherry, increase the heat to medium high, and cook until most of the liquid has evaporated. Add the broth. Tie the thyme sprigs, bay leaves, and peppercorns together in a cheesecloth sachet and toss it into the pot. Partially cover the pot, bring to a boil, immediately lower the heat, and simmer partially covered until the parsnips are soft enough to mash against the side of the pot with a wooden spoon. Remove from the heat and let cool for about 5 minutes. Discard the sachet. Using a stand or hand blender, puree the soup in batches, being sure to combine a mix of broth and solids in each batch. If you're using a stand blender, fill it no more than two-thirds full and be sure to vent the blender so the top doesn't pop off (either remove the lid's pop-out center or lift one edge of the lid and drape with a clean towel). Rinse the soup pot, return the blended soup to the pot, taste, and adjust the seasonings. If you're using the cream, add it now (if you're making the soup ahead, wait to add the cream until you reheat the soup just before serving). Garnish each bowl with croutons and a pinch of fresh thyme.