Preheat oven to 400°F. Prick potatoes all over with a fork, rub with oil, and season generously with salt and pepper.
Place potatoes directly on an oven rack and roast until soft when squeezed and skin is crisp, about 60–75 minutes. While the potatoes are cooking, prepare the yogurt sauce and lentil filling.
In a small bowl, combine yogurt, lemon juice, herbs and a pinch of salt and pepper, stir to combine, and set aside.
In small pot, combine lentils with 3 cups of water and a pinch of salt. Bring water to a boil, reduce to simmer and cook for 20 minutes or until lentils are tender. Once cooked, strain, rinse and set aside.
In a separate pan, warm oil on medium-high heat, add onion and cook for 2-3 minutes until tender.
Add garlic, bell pepper, mushrooms and chilli flakes and cook for another 4-5 minutes until all vegetables are tender.
Season with salt and pepper, add spinach and cook for another 2-3 minutes, stirring frequently, until spinach is wilted.
Add cooked and strained lentils directly to the pan of vegetables and toss to combine. Season with salt and pepper to paste.
Once the potatoes are cooked, remove them from the oven, slice them lengthwise down the centre and stuff each potato with a quarter of the lentil filling.
To serve, top each potato with a dollop of yogurt mixture and a sprinkle of parsley.