Adjust racks to top and middle positions and preheat oven to 475 degrees. Wash and dry produce.

If desired, peel potatoes; dice into ½-inch pieces. Remove outer leaves of cabbage; trim and discard root end. Quarter cabbage lengthwise. Cut broccoli into bite-size pieces if necessary. Zest and quarter lemon. Roughly chop parsley.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve ½ cup potato cooking liquid, then drain. 

Return potatoes to pot and add sour cream and 1 TBSP butter (2 TBSP for 4 servings); mash until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Taste and season with salt and pepper.

Keep covered off heat until ready to serve.

Arrange cabbage wedges, cut sides up, 3 inches apart on a baking sheet. Drizzle olive oil over each cabbage wedge; season with salt and pepper.

Roast cabbage on top rack until charred and tender, 25 minutes.

Once cabbage has roasted 10 minutes, toss broccoli on a second baking sheet with a large drizzle of olive oil, salt, and pepper. Roast on middle rack until browned and tender, 12-15 minutes.

While veggies roast, in a small pot, heat cream sauce base and cheese roux concentrate over medium-high heat; whisk to combine. Bring to a simmer, then cook, whisking frequently, until thickened, 1-2 minutes.

Add ½ cup water (1 cup for 4 servings); cook, continuing to whisk, until bubbling, 30-60 seconds more.

Turn off heat; whisk in Parmesan until fully incorporated.

Keep covered off heat until ready to use in next step.

Once cabbage is done, squeeze one lemon wedge (two wedges for 4 servings) over charred cabbage.

Return pot with fonduta to medium-high heat. Add lemon zest and juice from one lemon wedge (two wedges for 4); bring to a simmer, whisking constantly, until combined, 30-60 seconds. TIP: If fonduta becomes too thick, add splashes of water 1 TBSP at a time until desired consistency is reached.

Divide mashed potatoes, broccoli, and charred cabbage between plates in separate sections. Spoon fonduta over cabbage. Sprinkle with parsley and crispy fried onions. Serve with remaining lemon wedges on the side.