Caramelize your onions: Warmth butter and olive oil in a big, deep saute pan, ideally one which’s ovenproof, over medium warmth. Add the onions, toss to coat them in butter and oil and canopy the pot. Cut back the warmth to medium-low and let the onions slowly steep for 15 to twenty minutes — they don’t want your consideration. You need to begin the stirring course of sooner in the event that they’re browning or sticking to the pan. Uncover the pot, increase the warmth to medium-high (for now) and stir in salt — I begin with 2 teaspoons of kosher salt. Cook dinner onions, stirring each couple of minutes (you’ll be able to verify much less typically to start with, and should continuously stir later within the course of) till the onions are brown, comfortable, and candy, about 20 minutes extra. You’ll need to scale back the warmth to medium as soon as any water the onions emitted has cooked off.

Warmth your oven: To 375°F (190°C).

Assemble casserole: Add garlic to the onions and cook dinner, stirring, one minute. Add sherry, if utilizing, and cook dinner till it disappears. Add the farro and cook dinner for two minutes. Add lentils and 6 cups broth and thyme and convey the combination to a simmer. Season nicely with salt and pepper as wanted. In case your pan isn’t ovenproof, switch it at this level to a casserole dish.

Bake the casserole: Cowl the pan and bake, stirring about midway by way of, till the lentils and farro are tender, 40 to 50 minutes. If all the liquid has absorbed, both on the halfway by way of level or on the finish, add one other 1/4 to 1/2 cup splash of broth. We’re searching for a risotto-like looseness or mild “slosh” within the pan. Fish out thyme stems.

To complete: Sprinkle the dish with gruyere and broil till browned on prime.

Do forward: The dish retains fantastically within the fridge for 4 to five days; rewarm in a 350-degree oven. It’s also possible to put together the lentil and farro base a couple of days early, rewarm it (it can doubtless want extra broth at this level), and broil the cheese on prime proper earlier than serving.