Slice the potatoes into thick slices, with one medium-sized potato making about three slices. Place them in a pot with salted water and cook for about 20 minutes.
Add some oil to a pot and toss in the chopped onion and cut cabbage turnip pieces. Fry for about 8 minutes until the turnip pieces soften slightly and the onion is slightly browned.
For the next step, you can remove the turnip and onion from the pot, but you can leave them if you prefer. If left in, be more careful to prevent burning. Add the vegan butter and flour to the pot. Set the stove to medium-high heat and keep stirring.
After about 2-3 minutes, the flour and butter will combine into a thick paste and start bubbling. Once you see it slightly browning, it's time for the next step.
Carefully, while stirring the flour-butter mixture, add the soy milk and vegetable broth. If you don't have vegetable broth, use salt water or add water and half a vegetable broth stock cube. Keep stirring until there are no clumps, and the mixture develops into a creamy sauce.
Now add the fried onion, cabbage turnip, and frozen peas to the sauce. The sauce shouldn't be too thick as it still needs to cook on medium heat for about 15 minutes. If it seems too thick, add some water. Remember to season with salt and pepper throughout the process.
While the Cabbage Turnip en Rouge bubbles, start roasting your potato slices. After removing them from the pot, heat some oil in a pan and carefully place the slices in it. Season generously with salt and pepper. After about 4 minutes on medium to high heat, flip them once to brown the other side. Serve the roasted potatoes alongside the finished Cabbage Turnip en Rouge. Enjoy!