Grease a round cake pan with vegetable oil. Pour 1 cup (240 ml) water into the pot and nestle a steamer rack inside.
Blitz the egg yolks, whole eggs, condensed milk, evaporated milk, vanilla and salt in a blender at low speed until very smooth. Or whisk in a mixing bowl. Set aside until needed.
Cook the sugar, lemon juice and 2 tablespoons water in a small saucepan on the stovetop over medium-low heat, stirring occasionally with a silicone spatula. Once the sugar begins to dissolve, about 5 minutes, stir constantly until it turns into a clear syrup. Stop stirring and raise the heat to medium. Swirl the saucepan every few minutes, until the caramel is dark golden brown, 8 to 10 minutes. Very carefully pour the caramel into the prepared cake pan. Wearing heatproof mitts, quickly tilt the pan to evenly coat the bottom and up the sides a bit. Place on the steamer rack in the pot.
Skim any foam off the surface of the custard and pour through a fine-mesh sieve into the caramel-lined cake pan. Cover tightly with foil.
Lock the lid. Select PRESSURE COOK/MANUAL and set to LOW PRESSURE for 10 minutes. Make sure the steam release valve is sealed. Once pressurized (7 to 10 minutes), the cook cycle will start.
When the timer beeps, let the pressure release naturally for 10 minutes. Then quick release any remaining pressure. When the float valve drops, press CANCEL and open the lid.
Wearing heatproof mitts, remove the flan and set it on a hot pad. The center of the flan will still be a little jiggly.
Let it cool for 30 minutes. Refrigerate for at least 6 hours before unmolding and serving.
To unmold, run a butter knife along the edges of the cake pan. Place a plate over the pan and flip it over quickly. Place the plate on a flat surface and let the flan loosen itself from the pan.
Cut into 8 wedges and serve with whipped cream and/or fresh fruit.