Chop the cauliflower into 1 to 2 inch pieces. Working in 3 batches, pulse in a food processor with 4 to 5 quick one-second pulses per batch until it resembles rice grains. Do not over-process. Transfer to a bowl and set aside.

Heat the olive oil in a large wide-bottomed skillet over medium-high heat. Once shimmering, add the sliced kielbasa in a single layer. Cook undisturbed for 3 minutes, then stir occasionally for another 2 to 3 minutes until browned on all sides. Remove with a slotted spoon and set aside on a plate.

Reduce heat to just above medium. Add the butter to the remaining sausage fat in the skillet. Once melted and hot, add all the cauliflower rice. Cook uncovered for about 10 minutes, stirring occasionally, until softened and lightly golden in spots.

Add the lemon juice, salt, pepper, cayenne, and Dijon mustard. Stir well to combine. Add the chopped zucchini and stir. Cook uncovered for another 5 minutes, or until the cauliflower rice and zucchini are tender.

Add the frozen peas and reserved kielbasa back to the skillet. Stir and cook for 1 to 2 minutes until warmed through.

Turn off the heat. Spread the mixture evenly and sprinkle shredded cheddar over the top. Cover with a lid for 2 to 3 minutes to melt the cheese, or place under a broiler for about 1 minute until bubbling and lightly golden. Serve immediately.