Deb Perelman's Mushroom Bourguignon

From Food52 via Deb Perelman.
Ingredients:
2 Tablespoons olive oil
2 Tablespoons butter, softened
2 pounds 1/4-inch sliced portobello or cremini mushrooms
1 cup pearl onions (thawed if frozen)
1 carrot, finely diced
1 small yellow onion, finely diced
1 teaspoon fresh thyme leaves
Salt and freshly ground pepper, to taste
2 cloves garlic, minced
1 cup full-bodied red wine
2 cups vegetable broth
2 Tablespoons tomato paste
1 1/2 Tablespoons all-purpose flour
Egg noodles
Sour cream and chopped chives or parsley, for garnish (optional)

Heat the one tablespoon of the olive oil and one tablespoon of butter in a medium Dutch oven or heavy sauce pan over high heat. Sear the mushrooms and pearl onions until they begin to take on a little color, but the mushrooms do not yet release any liquid - about three or four minutes. It helps to do this in a few batches. Remove them from the pan and set aside.
Lower the flame to medium and add the second tablespoon of olive oil. Toss the carrots, onions, thyme, a few good pinches of salt and a several grinds of black pepper into the pan and cook for 10, stirring occasionally, until the onions are lightly browned. Add the garlic and cook for just one more minute.
Add the wine to the pot, scraping any stuck bits off the bottom, then turn the heat all the way up and reduce it by half. Stir in the tomato paste and the broth. Add back the mushrooms and pearl onions with any juices that have collected and once the liquid has boiled, reduce the temperature so it simmers for 20 minutes, or until mushrooms are very tender.
Combine remaining butter and the flour with a fork until combined; stir it into the stew. Lower the heat and simmer for 10 more minutes. If the sauce is too thin, boil it down to reduce to the right consistency. Season to taste.
To serve, spoon the stew over a bowl of egg noodles, dollop with sour cream and sprinkle with chives or parsley (optional).
Serves 4.