Preheat the oven to 400°F.
Add the ricotta to a medium bowl. Using a whisk, whip it with the lemon zest, salt, and pepper until light and fluffy. Set aside.
Add the garlic, balsamic vinegar, and olive oil to a shallow bowl. Season with salt and pepper. Add the asparagus and toss to coat.
Roll out the puff pastry and place it on a parchment-lined baking sheet. Use a knife to lightly score a 1-inch border around the pastry, being careful not to cut all the way through. This edge will puff during baking.
Spread the whipped ricotta over the center of the pastry, leaving the border clear. Arrange the asparagus in a single layer over the ricotta. Drizzle the remaining balsamic mixture over the asparagus.
Brush the edges with the beaten egg and sprinkle with flaky salt. Bake until the tart is deeply golden, and the bottom is crisp and golden, 25-30 minutes.
Meanwhile, make the hollandaise sauce: Fill a small saucepan with 1 inch of water and bring it to a gentle simmer, then lower the heat to minimum.
Find a heat-proof bowl that fits snugly on top without touching the water. Combine the egg yolks, lemon juice, and water in the bowl and place it over the simmering water. Whisk vigorously and constantly until the eggs turn pale yellow and thicken (about 1-2 minutes).
Add the butter cubes one at a time, whisking constantly, until fully melted and emulsified.
Slowly drizzle in the olive oil while whisking constantly until creamy and thick. For a more pourable consistency, continue whisking in olive oil until the sauce reaches your desired texture.
Remove the bowl from the heat. Whisk in cayenne pepper and salt.
Garnish the tart with thinly sliced radishes and chives. Serve with hollandaise sauce.