Make the rhubarb layer:

In a small saucepan, combine rhubarb, 2 tbsp sugar, and lemon juice. Cook over medium heat until the rhubarb softens and breaks down, about 8–10 minutes. Stir occasionally. Remove from heat and let cool slightly.

Make the custard:

Preheat oven to 325°F (160°C). In a saucepan, heat heavy cream and vanilla bean (if using) until just simmering. Remove from heat and let sit for 10 minutes. If using vanilla extract, add it after removing from heat.

In a mixing bowl, whisk egg yolks, 1/3 cup sugar, and a pinch of salt until smooth. Slowly pour the warm cream into the yolk mixture, whisking constantly to avoid curdling.

Assemble:

Spoon about 1–2 tablespoons of the cooked rhubarb mixture into the bottom of each ramekin (you’ll need 4–6 ramekins). Carefully pour the custard mixture over the rhubarb, filling each ramekin nearly to the top.

Place the ramekins in a baking dish. Pour hot water into the dish until it comes halfway up the sides of the ramekins.

Bake for 35–45 minutes, or until the custard is just set but still slightly jiggly in the center. Remove ramekins from water and let cool. Chill in the fridge for at least 4 hours or overnight.

Brûlée topping:

Just before serving, sprinkle 1–2 teaspoons of sugar evenly over each custard. Use a kitchen torch to caramelize the sugar until golden and crisp. Let sit for 1–2 minutes before serving.