As fancy as this dish looks, really the hardest part of it is finding the salsify: You can find it in specialty groceries in fall through spring. Scorzonera is a black-skinned variety. You can also buy salsify seeds and grow your own. There is no real substitute if you want to really have the three "oysters" in the dish, but a mashed potato croquette will look and taste fine. Use small oysters and small oyster mushrooms here, if you can. This is supposed to be an appetizer, or one course in a many-coursed meal.
Scrub the oyster shells well with a brush to remove all grit and dirt. Squeeze the lemon into a large bowl of cold water, and bring a pot of salted water to a boil. Peel the salsify (oyster plant, scorzon…you know what I mean…) one at a time and put each root into the water. When you’re done, cut each root into short discs and put into the boiling water.
Peel the potatoes, cut into pieces roughly the same size as the oyster plant and add them to the boiling water — this method ensures that both are soft at about the same time. Boil both veggies for 30 minutes or so; the salsify will take longer to soften than the potatoes.
Meanwhile, shuck the oysters over a bowl, letting the juice fall into the bowl. Set the oysters in a small bowl in the fridge. Strain the oyster juice through a paper towel into a small pot. Add 5 tablespoons of butter to the pot and melt over low heat.
When the salsify and potatoes are ready, mash with 1 tablespoon butter, thyme and a little of the milk or cream: You want the mash to be pretty solid, not super smooth or wet. Form the mashed salsify and potatoes into small discs. Dip into the beaten egg and then into the panko bread crumbs (you really do want panko breadcrumbs, which are very coarse; it’s a texture thing.) and be gentle, as this is not a very solid mass.
Get enough oil to reach up about 1/2 of an inch up the sides of a frying pan, and begin heating it to 350 degrees. You'll know the oil is hot enough when a pinch of flour sizzles immediately. Slide the croquettes gently into the hot oil. Fry until golden brown, about 2 minutes, then flip and cook the other side. When they’re all a lovely golden brown, drain the croquettes on a rack.
Heat another saute pan over high heat for 1 minute, then add the oyster mushrooms. Shake the pan constantly so the mushrooms don't stick until they sizzle and begin to release their water. Sprinkle salt over the mushrooms. When most of the water is gone, add the remaining butter and saute until the mushrooms begin to brown. When they are nicely browned, turn off the heat.
Turn the heat back onto the melted butter-oyster juice pot. When it begins to sizzle, turn off the heat and slip in the oysters. Remove after 1-2 minutes, or when the edges curl. Salt them lightly. To serve, place some oyster mushrooms down on the plate, then top with the clean bottom part of the oyster shell. Put the croquette in the shell, then top with the poached oyster and finally with a single borage flower.