Thoroughly rinse produce and pat dry. Peel potatoes, cut into 1" dice, and place in a medium pot with enough cool water to cover. Peel red onion and cut into 3/4" dice. Halve zucchini and yellow squash lengthwise, cut into quarters, then cut into 3/4" dice. Stem oregano and coarsely chop, reserving a couple leaves for garnish. Mince garlic.

Bring medium pot containing potatoes to a boil over high heat. Put a colander in the sink. Reduce heat to simmer and season with a pinch of salt. Simmer potatoes 15 minutes until fork tender. Drain in colander, return potatoes to pot, and combine with evaporated milk, 1 Tbsp. of butter, and parmesan cheese. Mash ingredients together with potato masher until smooth and combined. Season with a pinch of salt and pepper. Set aside.

Heat a large pan over medium-high heat with 2 tsp. olive oil. Add red onion and cook 3 minutes, stirring often, until just beginning to soften. Add zucchini and yellow squash and cook 6 more minutes. Add chopped oregano and garlic, cook 2 more minutes and add crushed tomatoes. Simmer ratatouille 10-12 minutes until vegetables are soft. Season with a pinch of salt and pepper.

Open sous-vide bag carefully and empty entire contents from bag into a medium pan. Add 1/2 cup water and bring to boil. Add lamb shanks and reduce heat to medium. Cover with a lid and simmer 10 minutes on each side, flipping them once halfway through. Internal temperature should reach 165 degrees. Remove lamb shanks to a plate and rest.

Leave sous vide juices in pan and heat over high heat. Reduce 5 minutes, until slightly thickened. Remove from heat and add remaining butter, swirling to combine. Taste and add salt and pepper if necessary.

Place a bed of potatoes in center of plate. Serve a lamb shank atop potatoes and add a serving of ratatouille next to them. Spoon pan sauce over the lamb shank and garnish with reserved oregano leaves.