In a large bowl place, the sliced Brussels sprouts and drizzle them with olive oil. Mix well and transfer them, cut side down on a cookie sheet.
Bake in the preheated oven for 20 minutes. They need to be brown and soft.
Meanwhile, whisk together mustard and mayonnaise. When the sprouts are done, mix them with the mustard mayo mixture.
Season with salt and pepper and add the parsley. Give it a last toss and serve.
Notes
A few tips and tricks will ensure your roasted Brussels sprouts turn out perfectly every time.
Start by choosing your Brussels bright green sprouts with the outer leaves intact and tight to each other. Firm when you gently squeeze them. Having them all the same size is beneficial for even roasting.
Trim the nubby ends and slice in half from the flat base through the top. You want that flat edge that will bake flush to the pan and caramelize.
Do not remove the core. It holds your leaves together, and during the roasting process, it will soften out and give you a great flavor.
Do not overcrowd the baking sheet, or they will produce steam when they roast and will never crisp up to your liking. If making them for a crowd, use two baking sheets. You will thank me for it later.
Bake at a high temperature to get crispy with golden edges, but not too high that the olive oil burns the baking sheet and creates that unpleasant bitter flavor.
But most importantly, do not disturb them when they are roasting.
When placed flat side down against the pan, Halved Brussels sprouts will steam the inside, making them perfectly golden and crispy on the outside and tender on the inside.
Giangi's Kitchen provides nutritional information, but these figures should be considered estimates, as a registered dietician does not calculate them.