Heat oil in a saucepan over medium-high heat. Add the shallot and garlic, cooking until softened, about 2-3 minutes.

Add the broth, lentils, Worcestershire sauce, and thyme. Bring to a boil, then reduce heat to low, cover, and simmer until the liquid is reduced by half but the lentils are not fully cooked, about 15-20 minutes.

Meanwhile, cover the potatoes in salted water in a separate saucepan and bring to a boil. Reduce heat to medium and simmer until the potatoes are tender, 10-15 minutes. Drain and return the potatoes to the saucepan off the heat. Use the back of a fork to smash them until smooth. Season with salt and pepper to taste and set aside.

When the lentils are ready, stir in the green beans, corn, carrots, and a pinch of salt. Cook until the lentils and vegetables are tender, about 8-12 minutes.

In a small bowl, whisk the cornstarch with a splash of cold water until a slurry forms. Stir the slurry into the lentils and simmer until thickened, about 3 minutes. Season with salt and pepper to taste.

Set the oven to broil on high. Transfer the lentil mixture to a baking dish and spread the mashed potatoes evenly over the top. Broil until the top is browned, 3-5 minutes (tip: broilers vary, so watch closely to avoid burning).