Preheat the oven to 400°F and line a baking sheet with parchment paper for easy clean-up.
Prep the Veggies: Cut the potatoes into small chunks and slice the peppers and red onion. You can add the veggies right to your prepped sheet pan. Cut the lemon in half and set it aside. Slice the sausages into large diagonal pieces and set aside.
Assemble the Seasoning Mixture: In a small bowl, mix together the lemon pepper seasoning, garlic powder, celery salt, oregano, and paprika.
Drizzle veggies with a generous amount of olive oil, hit with seasonings and mix well to combine. Add the two lemon halves, cut side up, drizzling them with some olive oil as well.
Get this roasting in the oven for 20 minutes.
Stir veggies around and then add in the chopped sausage. Put this back into the oven for 15 more minutes. Optional: finish under the broiler for the last few minutes to get a nice golden char.
Once the roasted lemon has cooled slightly, squeeze one half of the roasted lemon into a bowl with mayonnaise and whole grain mustard. Season with black pepper and a tiny pinch of salt. Stir well to combine.
Spread a spoonful of the creamy mustard roasted lemon sauce onto each plate, and then add the sausage, peppers, onions, and potatoes on top. Garnish with fresh Italian parsley and the extra roasted lemon half!