In a large pot over medium-high heat, add the olive oil, leeks, celery, and ½ teaspoon salt. Cook until softened and browned in spots, stirring occasionally, about 5 minutes. You want to get a little bit of a char to add lots of flavor.
Stir in the mushrooms and try to arrange them so that they are making contact with the bottom of the pot. Cook for 2-3 more minutes, stirring halfway, to soften and brown the mushrooms and leeks a bit.
Stir in the lentils, water, soy sauce, and remaining ½ teaspoon salt. Increase the heat to high to bring to a boil, then cover and reduce the heat to medium-low to maintain a simmer for about 20 minutes, stirring occasionally. Remove the lid and simmer for about 5-10 more minutes or until lentils are tender and liquid has reduced (you don’t want it watery, but definitely brothy if that makes sense!). You want the lentils to have a little bit of a bite, but you don’t want them crunchy or totally mushy.
Once you have the consistency you want, turn off the heat then stir in the lemon juice. Serve topped with a generous dollop of yogurt, fresh herbs, pickled red onions, and a side of buttered bread.