Leaders Celtic returned to form in the Scottish Premier League with a ruthless attacking display against Hearts.
Lassad gave them an early lead, beating Jamie MacDonald at his near post.
As Hearts passed up chances, Celtic extended their lead through Mikael Lustig's close-range strike and a Ryan Stevenson own goal.
Gary Hooper added a late fourth and Fraser Forster rounded off a fine individual performance by saving a Marius Zaliukas penalty.
The atmosphere inside Tynecastle was typically electric for such an occasion from the outset and the visitors' brighter start reflected their noisier support.
MacDonald had already saved a point-blank Kris Commons effort before Celtic took a 10th-minute lead in splendid fashion.
Beram Kayal spread the ball wide to the left and when Kris Commons curled a cross deep to the back post, Lassad swept a composed volley past the keeper.
Celtic had been showing none of the lackadaisical play that has often afflicted them in the SPL this season, but their defence went to sleep as Hearts should have recorded a swift response.
Stevenson swung a free-kick seemingly onto the head of the unmarked Callum Paterson, but the striker somehow contrived to miss the ball.
The cost of that miss was laid bare with 22 minutes gone as Celtic produced another slick passing move - this time down the right - and though Hearts repelled the initial danger, Lustig pounced on the loose ball to knock past MacDonald.
As formidable as Celtic were looking on the front foot, they still required the excellence of Fraser Forster to maintain their two-goal lead.
The England-squad keeper pushed away a ferocious Stevenson free-kick but it was his reaction to spring up and keep out Marius Zaliukas's header on the rebound that was truly impressive.
Hearts were competing doggedly but found themselves three goals down as the half-hour ticked by.
Charlie Mulgrew showed great appetite to prevent a loose ball from crossing the goal-line, firing it across goal and Stevenson, in his desperation to block, succeeded only in diverting it past MacDonald and into the net.
Hearts could justifiably argue that reflected harshly on the balance of play, but Celtic had delivered a masterclass in clinical finishing.
The home side again had cause to wonder what they had to do to score as Forster pulled off another fabulous save at the beginning of the second half, getting down to his right to turn a Kevin McHattie free-kick round the post.
The second period was unsurprisingly less frenetic than the first, with Hearts unable to make a significant impression on the Celtic rearguard.
They did claim for a penalty - not for the first time in the match - when the ball bounced up onto the arm of Efe Ambrose, but referee Willie Collum adjudged it not to have been deliberate.
That was with just over a quarter-of-an-hour to play and shortly afterwards the Hearts substitute Mehdi Taouil volleyed into the arms of Forster.
As full-time approached, MacDonald saved well from Commons, but as the ball was played back into the box by Mulgrew, Hooper couldn't miss from inside the six-yard box.
Hearts were handed an opportunity to notch a consolation goal from the penalty spot when Kelvin Wilson brought down Stevenson, but the impeccable Forster guessed correctly to save Zaliukas's spot-kick.