To prep thighs, lightly salt and pepper and sous vide for 8 hours at 150 degrees, or place in a baking dish and cover with foil, place in oven at 150 for 8 hours.
To make gravy, add a thin layer of olive oil to a large skillet. Add onions and lightly salt and pepper. Sear on medium heat. Once slightly seared, turn heat to medium-low to caramelize onions, which could take upwards of 45 minutes.
Once onions are caramelized or are close, add 1 tablespoon salted butter and garlic and stir in. Turn heat to medium. Simmer for a couple of minutes, then add Guinness to deglaze skillet.
After stirring in the Guinness, add balsamic vinegar. Stir for another minute then add beef stock, browning sauce, brown sugar, kosher salt, ground black pepper, and mustard, along with sprigs of thyme and sage. Reduce for 15 to 30 minutes, until beef stock is half the original amount.
Once the beef stock has reduced to half, remove thyme and sage sprigs and throw them away. Create a cornstarch slurry by adding 2 tablespoons cornstarch to a bowl and drip in cold water until it's the consistency of a paste. Add to gravy mix and stir in to thicken. Reduce to very low heat until ready to serve.
To make colcannon, wash and cut potatoes into chunks. Add to a large pot with water and bring the water to boil. Boil until the potatoes are fork-tender, around 7 to 10 minutes.
Drain the potatoes in a colander, and lightly salt and pepper. Add the pot back to the burner and reduce heat to medium-low. Add 6 tablespoons of salted butter to the pot along with kale leaves. Lightly salt and pepper once more and stir until leaves are wilted and cooked in butter. Once kale is cooked and wilted, add sliced green onions and cook for another minute.
Add potatoes back to the pot and mash in with kale. Add 1 cup heavy whipping cream and stir in. Add more cream if you want a creamier texture. Salt to taste.
To cook pheasant, heat a grill or skillet to 500 degrees. Spray both pheasant breasts and thighs with cooking oil and add to grill or skillet and sear both sides evenly. Remove once the breasts are 160 degrees internal and skin on both the thighs and breasts is crispy.
To serve, add pheasant to a plate along with colcannon and top with gravy. Garnish with sliced scallions.