8 ounces pearl onions, peeled (about 12 to 15 onions)
8 ounces cremini mushrooms, halved if large (about 4 cups)
1 tablespoon extra-virgin olive oil
Chopped flat-leaf parsley for garnish
Season beef with 2 teaspoons salt and 1/2 teaspoon pepper. Set aside for at least 30 minutes at room temperature or chill in the refrigerator for up to 24 hours.
In a large Dutch oven or heavy-bottomed pot with a tightfitting lid, cook lardons over medium-low heat until fat is rendered and lardons are browned and crisp, about 10 to 15 minutes.
With a slotted spoon, transfer to a paper towel-lined plate. Reserve fat in pot.
Heat oven to 350 degrees. Raise heat under pot to medium-high and cook until fat starts to smoke.
Lay half of the beef cubes in a single layer in the pot, leaving space between pieces, and cook until well browned on all sides, 10 to 15 minutes. Repeat with remaining beef. Removed beef from the pot and set aside.
Add onion, carrot and remaining 1/4 teaspoon salt into the pot with the beef fat and cook, stirring occasionally, until soft, about 10 minutes.
Stir in garlic and tomato paste, and cook for 1 minute.
Add flour and cook, stirring frequently, for 1 minute then add wine, bay leaf and thyme, scraping up brown bits at the bottom of the pot.
Add browned beef and half of the cooked lardons back to pot, cover, and transfer to oven. Let cook until beef is very tender, about 1 1/2 hours, turning meat halfway through.
Meanwhile, in a large skillet set over high heat, combine pearl onions, mushrooms, 1/4 cup water, olive oil and a pinch each of salt, pepper and sugar.
Bring to a simmer, cover and reduce heat to medium, cooking for 15 minutes. Uncover, raise heat to high, and cook, tossing frequently, until vegetables are well browned, 5 to 7 minutes.
To serve, scatter onions and mushrooms and remaining cooked lardons over stew, then top with parsley.