Heat the oven to 420 degrees. Give the oven time to become fully hot so the cauliflower starts sizzling as soon as it reaches the pan.

Break the cauliflower into even-sized florets and place them in a large bowl. Keep the core stems similar in thickness so smaller pieces do not darken before larger ones soften.

Whisk the mayonnaise, Dijon Mustard, salt, and pepper in a small bowl until smooth. The mixture should look evenly blended with no streaks of mustard remaining.

Add the seasoned mayonnaise mixture to the cauliflower and toss until every floret is coated. Look for a thin, even layer over the branching surfaces rather than large pools at the bottom of the bowl.

Arrange the coated florets in one layer on a roasting pan, or set an oven-safe cooling rack on a cookie sheet and spread them across the rack. Leave space between pieces so hot air can reach their sides and edges.

Roast for 25 to 30 minutes, stirring every 10 minutes. The cauliflower is ready when the tips are deep golden and a fork meets only gentle resistance in the thicker stems.

Move the hot cauliflower to a bowl and toss with melted butter, Parmesan, lemon juice, salt, and pepper. Serve while warm, when the cheese is fragrant and the browned edges still have their best texture.