Lentil Shepherd's Pie

900 g russet potato, peeled and quartered

1 yellow onion, finely chopped

2 carrot, peeled and diced

2 celery, diced

3 garlic, minced

2 thyme, leaves stripped

60 g unsalted butter, divided

120 ml whole milk, warm

120 g frozen peas

30 ml olive oil

180 g red lentils, rinsed

750 ml vegetable stock

30 g tomato paste

15 ml worcestershire sauce

1 bay leaf

12 g kosher salt, divided

3 g black pepper, freshly ground