Preheat oven to 425 degrees. Line a 12" x 17" baking sheet with parchment paper.
Add all sauce ingredients to the blender and blend until smooth. The final result looks a lot like a very smooth peanut butter.
Heat a large skillet on the stove on medium-high heat.
Add some oil to the pan and place your chicken thighs in pan.
Brown. Flip. Brown. When both sides have some nice coloring remove them from the skillet and place top side down on parchment paper lined pan.
Slice potatoes into chunks,. Place the potatoes into a bowl and add 2 heaping spoons of your sweet mustard sauce. Stir to fully coat the potatoes. Add the coated potatoes in a single layer to the pan beside the chicken, leaving space for brussels sprouts to be added later.
Sprinkle salt and pepper over potatoes and chicken thighs. Sprinkle italian seasoning over potatoes. Give the potatoes a quick stir and spread back into an even layer.
Use a spoon or spatula to spread half of the remaining sweet mustard sauce on chicken thighs.
Place baking sheet in the oven and bake for 15 minutes.
While your chicken and potatoes are baking, trim the bottom of each brussels sprout and peel away the first layer of leaves.
Remove the chicken and potatoes from the oven. Flip the chicken and coat the top with the remaining sweet mustard sauce.
Add brussels sprouts in a single layer to the sheet. Drizzle with a little bit of olive oil and salt.
Place back in the oven and bake for another 25 minutes.