Leo's 25-yard bullet was worthy of any footballing stage but, instead, 3,093 fans saw it whistle home to hand us a 2-0 lead.
A team including seven players eligible for our Under-19s and only Leonhardsen with any Premiership experience tore apart Third Division opponents.
Lee Barnard continued his fine pre-season form with the opening goal, Rohan Ricketts curled in his first goal in Spurs colours and John Jackson capped an outstanding night at full-back with the fourth goal.
But forget the scoreline, the quality of football was there for all to see and apart from two spells, the gulf between the sides was huge.
The first half was impressive stuff as we controlled possession and, barring a couple of efforts late in the half, it was one-way traffic.
It took just seven minutes to carve out the first chance as Ricketts fed Bortolozzo, who reached the byline but saw Barnard blast his cut back into the flats behind the stand.
We took the lead three minutes later. Sutton pulled wide right, a tactic used throughout the half and his pinpoint cross was flicked home by Barnard, who managed to get across his defender.
It was soon 2-0 as Leonhardsen pounced on a poor clearance from Jackson's cross, steadied and rifled into the top corner from 22 yards.
The next 20 minutes saw wave after wave of attacks and a host of half-chances.
Sutton's low curler was saved comfortably by Flahaven, Jackson's delicious low cross evaded everyone and Barnard flicked Galbraith's corner just wide.
Southend's best spell followed just before the break as first Jones headed Searle's free-kick just wide and, two minutes later, the bustling Jenkins played a couple of one-twos only for Kelly to throw himself in front of his shot.
The half ended with us in the ascendancy as Leonhardsen laid the ball into Bortolozzo and the Brazlian's first-time effort flashed just wide.
The second half started in similar fashion as Jackson's free-kick was superbly tipped onto the inside of the post by Flahavan.
But the home side came storming back with their best period so far. Jones and ex-Spurs trainee Maher combined after 55 minutes, forcing Jalal to save with his feet from close range.
The goal was only two minutes away though as Belgrave's powerful header crashed off the underside of the crossbar and just over the line.
Kelly headed wide when well placed but it was Jalal making the saves midway through the half as he first smoothered Selley's effort from Maher's corner low to his left and then parried well again with his feet as Searle poked towards goal after a scramble.
Just as the United fans were baying for an equaliser, we struck at the other end.
With 70 minutes on the clock, Snee clipped the ball into Ricketts who was allowed to pick his spot, unchallenged, into the far corner from 12 yards.
Jalal then produced the save of the night - a hat-trick of saves with his feet - to block Jenkins' close-range volley from Beard's hanging cross.
Jackson then capped off a night to remember, timing his run superbly to meet Snee's cross and head home low into the corner from a tight angle with six minutes left.
Sutton still had time to curl onto the post but by then, the night's work had been more than done.
Southend United: Flahavan, McSweeney (Beard, 56), Cort, Whelan, Searle, Clark (Thurgood, 71), Minton (Selley, 56), Maher, Jenkins, Belgrave, Jones (Webb, 63).
Spurs XI: Jalal, Hughes, Jackson, Kelly, Henry, Bortolozzo (Galbraith back on, 84), Leonhardsen (Thomas, 71), Ricketts, Sutton, Barnard (Snee, 45), Galbraith (Marney, 45).
Oyvind Leonhardsen's stunning strike was the highlight of an excellent's night's work from Colin Calderwood's men at Roots Hall.