Begin with the potatoes. Peel the potatoes and cut them into large, even sized pieces. Put them in a large pot and cover with cool water by an inch or so. Bring the water to a boil and cook until the potatoes are fork tender. Drain the potatoes and return them to the pot.
Mash the potatoes with a potato masher and add the butter and milk. Continue to mash until they are creamy and to the texture of your liking. Season with salt and pepper and set aside.
Start the filling by heating a large saute pan over medium heat. Add the oil, onion, carrots, garlic, rosemary and thyme.. Saute for 5 minutes, or until the onions star to caramelize.
Season the lamb well with salt and pepper. Add the lamb to the pot and brown on all sides, for roughly 4-5 minutes.
Add the tomato paste, Worcestershire, and flour to the pan and stir well to evenly distribute. Cook another 3 minutes. Add the Guinness and stir with a whisk to combine. Bring to a boil and then turn off the heat. Add the corn and peas (still frozen) to mix and gently stir just to combine. Taste and season with salt and pepper.
Spray each of the four ramekins (or a 9 x 9 baking dish) with cooking spray. Evenly distribute the filling among for four ramekins. Top with mashed potatoes, using a spoon to give them a rustic, wavy look in the bowls.
Bake the shepherd's pies for 20 minutes on the middle rack in your oven. After the 20 minutes, turn the oven onto the LOW broil setting. Bake another 5 minutes, or until the potatoes are a nice golden brown with some crispy edges. Top with fresh thyme to serve.
Recipe by Cooking in Red Socks at http://www.cookinginredsocks.com/2014/03/13/lamb-and-guinness-shepherds-pie/