  Quantum dots with large Thouless number $g$ embody a regime where both
disorder and interactions can be treated nonperturbatively using large-N
techniques (with $N=g$) and quantum phase transitions can be studied. Here we
focus on dots where the noninteracting Hamiltonian is drawn from a crossover
ensemble between two symmetry classes, where the crossover parameter introduces
a new, tunable energy scale independent of and much smaller than the Thouless
energy. We show that the quantum critical regime, dominated by collective
critical fluctuations, can be accessed at the new energy scale. The
nonperturbative physics of this regime can only be described by the large-N
approach, as we illustrate with two experimentally relevant examples.
