Combine the cream with 1/2 cup of the sugar in a medium saucepan over medium heat. Bring to a strong simmer just so tiny bubbles start to appear around the edges of the pan; don't let it boil.
Meanwhile, in the bowl of a stand mixer using the whisk attachment, whisk together the egg yolks and remaining 1/2 cup sugar for about 1 minute. Whisk in the tequila.
Now, with the mixer on medium low, very slowly drizzle in the warm cream using a metal measuring cup to scoop out small portions at a time. Drizzle it in so that the cream runs down the side of the bowl, reducing the impact of the hot cream hitting the eggs. Keep whisking!
After all of the cream is added, turn off the mixer. Pour the mixture into the top of a double boiler (or a glass bowl fitted over a saucepan of simmering water). Cook in the double boiler over medium heat, stirring gently but constantly as the egg/cream mixture slowly thickens, from 8 to 15 minutes. Watch the mixture: if it starts to really thicken up, remove it immediately from the heat. You want this to be a thick, pourable cream rather than a thick, heavy pudding.
Prepare a large bowl with ice. As soon as you remove the double boiler or glass bowl from the heat, set the bottom of it in the bowl of ice to stop the cooking process. Be careful not to let ice or water drip into the cream mixture. Stir in the lime zest and continue stirring to cool. If you need to use the cream right away, keep it in the ice bath. Or transfer it to a container, cover and refrigerate for several hours.
When ready to serve, fill bowls with the berries and spoon the cream on generously.