Vegan Irish Mushroom and Stout Stew

1 Heat the olive oil in a large soup pot on medium high. 
2 Add the onion, garlic and leek to the pot. Sauté, reducing heat as needed, until the onion is translucent. About 2-3 minutes. 
3 Now add the mushrooms, and stir to combine everything. Sauté the mushrooms until they release their liquid and then they start to brown. Season with a few pinches of salt and pepper. This will take about 10-12 minutes. 
4 Then add the carrots, celery and cabbage. Sauté for another 1-2 minutes.
5 Pour in the stout and stir to combine. Simmer until a lot of the stout has been absorbed, 5-10 minutes. 
6 Add the vegetable broth, thyme, bay leaves, agave and a few pinches of salt and pepper. Stir to combine everything. 
7 Bring to a simmer, reduce heat to low and cover. Simmer for about 15 minutes.
8 Whisk together the cornstarch and water in a small bowl, then add it to the pot and stir to combine.
9 Put the cover back on and simmer for another 5-7 minutes or until the stew has thickened slightly. It will thicken more as it cools. 
10 Remove the thyme sprigs and bay leaves. Then taste and adjust seasonings. 
11 Serve over mashed potatoes if desired!