Pre-heat oven to 375 degrees.
Heat a medium sized pot to medium heat, sear whole cauliflower in butter, rotating until even color all over.
Place pot, uncovered, in oven for 30–45 minutes.
Remove cauliflower from pot and let rest on a separate plate, discard any browned butter left in the pan.
Add chicken stock, pine nuts, olives, lemon zest, mustard greens and cook until wilted.
Check for seasoning and finish with 1/2 squeezed lemon juice.
Serve roasted cauliflower on the bed of mustard greens, and garnish with lemon.