Preheat the oven to 425°F.
Place the beets in a large pot, cover by 2 inches with cold water and add a pinch of salt. Bring to a simmer, reduce the heat, and cook until fork-tender, about 45 minutes. Drain and run under cold water. When cool enough to handle, rub the beets to remove the skin and slice into wedges. Season with salt and pepper.
Bring 2 cups of salted water to a boil with the bay leaf in a small saucepan. When the water boils, add the lentils, bring to a simmer, reduce heat to low and simmer for 30 minutes or until tender. Drain.
On a baking sheet, toss the carrots with the olive oil and salt. Roast until tender, about 7 minutes.
Place the branzino on a baking sheet, drizzle with olive oil and sprinkle liberally with salt and pepper, including the cavity.
Heat a large ovenproof skillet over high heat. Add the branzino and cook for 3 minutes per side, until the skin is crisp. Transfer to the baking sheet and roast for 10 minutes, until cooked through.
Add the butter, miso and sugar to a mixing bowl and beat on high until light and fluffy. Gradually add in the mirin, oil and hazelnuts. Season to taste with salt.
For the pea tendrils, heat the oil in a large skillet over medium heat. Add the onions, garlic and salt and sauté until tender, about 5 minutes. Add the pea tendrils and cook for 30 seconds. Remove from heat and serve immediately.
To serve, divide the lentils between plates. Top with golden beets, roasted carrots, branzino, pea tendrils and miso butter and a lemon wedge on the side.