This custardlike dessert is a traditional Spanish sweet.
Preheat oven to 325 degrees. In a small saucepan, bring 1/2 cup sugar and the water to a boil over medium-high heat. Without stirring, cook until syrup is a rich amber color, about 8 minutes; brush down sides of pan with a wet pastry brush to prevent crystals from forming. Pour into an 8-by-2-inch round tart pan, swirling to coat bottom evenly. Set aside.
In a medium saucepan over medium-high heat, bring half-and-half and ground almonds just to a boil; remove from heat. Let steep, covered, 20 minutes. Strain through a fine sieve into a clean saucepan, pressing with a rubber spatula to extract liquid; discard solids.
Whisk remaining 2/3 cup sugar and salt into mixture in saucepan; bring to a simmer over medium-high heat. Whisk together eggs and yolks in a medium bowl. Whisking constantly, gradually add half-and-half mixture. Stir in the amaretto. Pour through a fine sieve into prepared pan; place in a roasting pan.
Cover the flan with foil; poke several holes in foil. Place pan in oven; fill it with boiling water to reach halfway up sides of flan. Bake until just set, 50 to 55 minutes. Transfer to a wire rack to cool completely. Cover with plastic wrap; chill overnight. Run a thin knife around edge of flan; invert onto a serving plate.