This delicious sauce is perfect for dressing vegetables, flavoring mashed potatoes, or enhancing any dish's flavor. Quite sophisticated, it contains more than 10 ingredients. However, if you want something simpler, omit the ones where you see "optional" marked. It will still be tasty. The advantage of a demi-glace is to give a tasty flavor, umami, to your preparations. Your friends will love the meaty taste it evokes. You can cut the ingredients' quantity in half, but you might as well make a large amount of sauce if it's going to take that long to prepare. What's more, no one will hold it against you if you want to perfume your kitchen more than once.
Using a mandolin or vegetable peeler, finely chop or grate the vegetables and transfer them to a large and deep baking sheet.
Add the tomato paste and, if desired, the kombu. Stir to combine.
Toss the vegetables with oil to prevent them from sticking. Transfer to the oven and roast for about 45 minutes. Check them every 20 minutes or so, stirring and turning as needed to keep the edges from burning.
Add water to the vegetables and return to the oven for 30 to 40 minutes.
Filter the broth into a saucepan. Optionally, scrape and peel off the remaining vegetable pieces on the baking sheet with the wine, then filter carefully into an empty corner of the sieve to not lose the wine in the pile of vegetables. (A thin layer may form at the surface when let to cool for a few minutes.)
Over medium heat, reduce broth to about half of its original volume. Remove from heat.
Whisk the the pectin in the demi-glace. Incorporate the guar gum until there is no lumps and the liquid thickens.
Serve with your favorite dishes.
Notes
Vegetables: you can add as many vegetables as you want. Ones that wilt easily will do well. Make sure there are saltier than sweet veggies; otherwise, the sauce may be syrupy and sickening.
Mushrooms: shiitake really add flavor. Dehydrated mushrooms taste much more pronounced than fresh ones, so if you use those, halve the amount and soak them in water to rehydrate them. If you don't like this kind of mushroom, replace them with white or coffee mushrooms or four portobello mushrooms.
Tomato paste works well here to provide a concentrated tomato flavor. It can also act as a thickener after the liquid has been filtered. However, it will change the taste (more acid and more umami). It can also be replaced by Passata. Sauté the tomato paste to alleviate its flavor.
Salt: There is little salt in this recipe, but add some if you don't include enough salty ingredients like celery, celeriac, or kombu.
Thickeners: If you think you'll rarely be using guar gum or xanthan gum in the future, you can opt for cornstarch or unbleached all-purpose flour instead (much cheaper, too).
Variants
For an easy vegetable demi-glace, you can omit the cauliflower, celery root, and/or beet.
Conservation
Placed in an airtight container, this vegetable demi-glace will keep for up to two weeks in the refrigerator. For longer conservation, keep 1 cup in the fridge and freeze the rest in small releasable bags.